National, state and territory population methodology

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Reference period
March 2021

Estimated resident population


Australia's estimated resident population (ERP) includes all people who usually live in Australia (regardless of nationality, citizenship or visa status), with the exception of people present for foreign military, consular or diplomatic reasons.

Geographic coverage

This data covers Australia and its states and territories, as defined by the Australia Statistical Geography Standard 2016.

ERP for Other Territories is available from September quarter 1993 onwards. Before then, Jervis Bay Territory was included in the ACT estimate, while Christmas Island and Cocos (Keeling) Islands were excluded from ERP. Norfolk Island has been included in Other Territories since 30 June 2016. Prior to this, the population of Norfolk Island was not part of Australia’s ERP.

The populations of Australian external territories are updated annually to fulfil the Commonwealth Electoral Act 1918, but are not part of Australia’s ERP. These external territories are:

  • Territory of Ashmore and Cartier Islands
  • Coral Sea Islands Territory
  • Australian Antarctic Territory
  • Territory of Heard and McDonald Islands


The latest ERP is based on adjusted 2016 Census counts, updated with quarterly estimates of births, deaths, overseas and interstate migration. Further information on each component can be found below.


Quarterly ERP is calculated by taking the population estimate at the start of the quarter and adding natural increase (births minus deaths), net overseas migration and (in the case of state/territory populations) net interstate migration. These calculations are done for each age-cohort and sex. This is known as the cohort component method, and uses the demographic balancing equation.

The demographic balancing equation is:

\(P_{t+1} = P_{t} + B - D + NOM + NIM\) where:

\(P_{t}\) = the estimated resident population at time point \(t\)
\(P_{t+1}\) = the estimated resident population at time point \({t+1}\)
\(B\) = the number of births occurring between  \(t\)  and \({t+1}\)
\(D\) = the number of deaths occurring between \(t\) and \({t+1}\)
\(NOM\) = net overseas migration occurring between \(t\) and \({t+1}\)
\(NIM\) = net interstate migration occurring between \(t\) and \({t+1}\)

At the national level, net interstate migration is zero.

Revision status

The status of quarterly ERP data changes over time from preliminary to revised to final as new component data becomes available. Preliminary ERP is updated every quarter due to revisions to the component data for earlier quarters. ERP gets marked as revised once it can be expected not to change again until the final update, 22 months after the next Census.

The table below shows the current status of ERP and the components of population change. For explanation of the differences between preliminary, revised and final status, see explanatory notes for each component.

QuartersBirths and deathsOverseas migrationInterstate migrationEstimated Resident Population
Sep.1991-Jun. 2016 Final Final Final FINAL 
Sep. 2016-Mar. 2020RevisedFinalPreliminaryREVISED 
Jun. 2020RevisedRevisedPreliminaryPRELIMINARY - updated due to revised component data 
Sep. 2020 - Dec. 2020Preliminary RevisedPreliminaryPRELIMINARY - updated due to revised component data 
Mar. 2021Preliminary PreliminaryPreliminaryPRELIMINARY

Rebasing method

The 30 June ERP in a Census year is calculated by:

(1) adjusting Census counts of Australian usual residents to include Australian residents temporarily overseas and adjust for people missed or counted twice in the Census (based on Post Enumeration Survey results)

(2) removing any births, deaths and migration movements that happened between Census night to 30 June.

This new ERP becomes the base for quarterly estimate going forward from that point.

For further information see Australian Demographic Statistics, December quarter 2017, Feature Article: Final Rebasing of Australia's Population Estimates using the 2016 Census.

The differences between this new Census-based estimate (known as rebased ERP) and the quarterly component-based estimate (known as unrebased ERP) is called ‘intercensal difference’. All quarterly estimates since the previous Census are revised, to incorporate the intercensal difference by age-cohort and sex evenly across the 5-yearly period between Censuses.

Intercensal difference may result from differences in the start population estimate, the end population estimate and/or in the quarterly estimates of births, deaths or migration. It is not possible to attribute intercensal difference to a particular source. For further information see Population Estimates: Concepts, Sources and Methods, 2009.

Births and deaths


Births and deaths estimates use information provided by each state/territory registry of births, deaths and marriages.

Preliminary method

Preliminary estimates are based on the number of births and deaths registered in the target quarter. Counts based on date of occurrence are incomplete at this time, so date of registration is used as an initial proxy until the estimates are revised with more extensive date of occurrence data.

Revised method

Revised estimates are based on the number of births and deaths that occurred in the target quarter, and account for registration lag.

Registrations may be lagged due to: 

  • Delays in the informant submitting the required information to the registry
  • Delays in processing at the registry
  • Delays in the ABS receiving complete information from the registry

Revised estimates include births or deaths that occurred in the target quarter and are registered from 3 to 6 quarters later. This directly measures most of the lag.

The period of lag directly measured depends on the quarter being estimated, because four quarters of estimates (a financial year) are revised at once.

The directly measured period of lag is: 

  • Up to 6 quarters for Sep quarter estimates
  • Up to 5 quarters for Dec quarter estimates
  • Up to 4 quarters for Mar quarter estimates
  • Up to 3 quarters for Jun quarter estimates 

Some births and deaths are assumed to have occurred in the target quarter, but not registered as the time of revising. This represents unobserved lag at the time of estimations, and that such occurrences will assumedly be registered in the future.

To account for unobserved lag, the most recently observed lags of more than 3 to 6 quarters are assumed to be true for the target quarter. The proportion of records that were registered in the corresponding quarter of the most recent year, and occurred more than 3 to 6 quarters earlier, are calculated. Expansion factors are derived from this and applied to the counts of registrations occurring in the target quarter.

The expansion factor must be greater than or equal to 1. For each state/territory \(s\) and quarter \(x\) and lag period \(y\), it is calculated as:

\(f_{s,x}=\frac{(\text{Count of total events registed in s, x})}{(\text{Count of events registered in s, x and within y quarters of occurring})} \)

The revised estimate is calculated as:

\(E_{s,x}=f_{s,x}\times(\text{Count of registrations occurring in s, x})\)

Birth and deaths estimates may be revised for a final time after each Census, when intercensal population estimates are finalised. 


Births and deaths data in this release are not necessarily consistent with data in Births, Australia and Deaths, Australia. The estimates in this release use registration counts by quarter of registration (preliminary) or by quarter of occurrence plus modelling (revised). The data in Births, Australia and Deaths, Australia are compiled only from registrations provided to the ABS, with no adjustments, and presented by year of registration, except for tables showing registrations by year and month of occurrence.

Current issues

March 2021: Victoria. COVID-19 restrictions have led to longer than usual delays in birth registrations in Victoria.

December 2020: Victoria. Lower than usual registrations in the second half of 2020 for reasons including COVID-19 shutdowns and subsequent behaviour.

December 2019: Victoria. As a result of joint investigations between the ABS and the Registry additional death registrations from 2017, 2018 and 2019 were identified that had not previously been provided to the ABS. An issue associated with the Registry's previous processing system (replaced in 2019) has resulted in delays to the provision of some death registrations to the ABS. Approximately 570 additional deaths registrations have now been included in the December 2019 ERP, with the deaths being added across the five previous quarters. The remaining additional deaths (approximately 2200) occurred in 2016-17 and 2017-18, and will be included as part of the final revision of intercensal components in June 2023.

Overseas migration


For the purposes of overseas migration, a person is added to Australia’s population if they have been (or expect to be) in Australia for 12 months or more. Likewise, an Australian resident is removed from the population if they leave Australia for 12 months or more.

The 12 months does not have to be continuous and is measured over a 16-month period.


Overseas migration data is sourced from Australian Government Department of Home Affairs processing systems, visa information, and incoming passenger cards.

Preliminary method

Preliminary estimates of overseas migration are required less than six months after the reference quarter for the production of quarterly ERP. At that time, complete traveller histories for the 16 months following a reference quarter are not available. These estimates are modelled, based on the behaviour of similar travellers one year earlier. The characteristics defining similar travellers are: age, country of citizenship, direction of first and last movement in the reference quarter, initial ERP status, time spent out of Australia, and visa group.


Preliminary estimates are subsequently revised each quarter until they are made final. These revisions incorporate all available and the most up to date overseas movements data. 

Final method

Revised overseas migration estimates are based on observed traveller behaviour. Once 16 months has elapsed, the source data determines each travellers’ actual duration of stay in or out of Australia and consequently their inclusion or exclusion from the ERP.

Historical changes

From the March quarter 2020, the COVID-19 pandemic caused extreme disruption to travel patterns and volumes to and from Australia.  To increase the sensitivity of estimates to recent international travel restrictions, from December quarter 2019 all overseas migration estimates which had not already been made final have been revised each quarterly processing cycle. 

Prior to this, due to the stability of travel patterns and the accuracy of the preliminary estimates, overseas migration estimates were ordinarily revised only once. This occurred one year after production of the preliminary estimate when they were made final.

In 2018 a new method of calculating the preliminary estimates was introduced, in response to outgoing passenger cards being discontinued. The new method proved to be more accurate predictor of traveller behaviour, and was backdated to September quarter 2011. For further information see the Information Paper: Improvements to estimation of net overseas migration, Mar 2018.

Prior to September quarter 2006, foreign diplomatic personnel and their families were included in migration estimates, as the previous method was unable to exclude them.
The rule of 12 months out of 16 has applied since September quarter 2006. Prior to that, migration was measured using a continuous 12-month period. Consequently, this point marks a break in series and overseas migration estimates from earlier periods are not directly comparable. For further information on the 12/16 month rule see the Technical Note
'12/16 month rule' Methodology for Calculating Net Overseas Migration from September quarter 2006 onwards in Migration, Australia, 2008-09.

Interstate migration


Quarterly interstate migration cannot be directly measured and is estimated using administrative data. Interstate migration is estimated based on a combination of Census data (usual address one year ago), Medicare change of address data (provided by Services Australia), and Department of Defence records (for military personnel only).

Preliminary method

Medicare address information is the basis of the model as its scope and coverage is the highest quality out of all available administrative data sources. There are some people who are part of ERP but are not covered by Medicare, such as certain temporary visa holders. For others there is a time delay from when they move residence to when they update their address details with Medicare. To account for these issues, factors are applied to calibrate the data to the interstate migration patterns we see every five years in Census. These factors are applied by age, sex, state and move type (arrival or departure).

As many Defence force personnel do not interact with Medicare, and also have high rates of interstate migration, Defence movements data is also used to supplement the Medicare data.

Medicare data received for one quarter is used to estimate interstate migration for the previous quarter. This assumes that on average the time between a person moving house and registering their change of address with Medicare is three months.

Revised method

Interstate migration estimates are revised following each Census to incorporate the most accurate data from all sources. For information on the most recent revised method, see 2016 Census Update of the Net Interstate Migration Model, 2011-2016.

Geographical coverage

Preliminary interstate migration estimates for Other Territories are not available. These movements are included in the data for New South Wales (Jervis Bay Territory and Norfolk Island) and Western Australia (Christmas Island and Cocos (Keeling) Islands).


The ABS collects statistical information under the authority of the Census and Statistics Act, 1905. This requires that statistical output shall not be published or disseminated in a manner that is likely to enable the identification of a particular person or organisation.

To guard against identification or disclosure of confidential information, sensitive data cells may be suppressed or subject to small random adjustments. Suppressed cells are marked as not available for publication (np) but are included in totals where applicable. In these cases, data may not sum to totals.

ABS statistics draw extensively on information provided freely by individuals, businesses, governments and other organisations. Their continued cooperation is very much appreciated: without it, the wide range of statistics published in the ABS would not be available. Information received by the ABS is treated in strict confidence as required by the Census and Statistics Act 1905.


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12/16 month rule

Under a '12/16 month rule', incoming overseas travellers (who are not currently counted in the population) must be resident in Australia for a total period of 12 months or more, during the 16 month follow-up period to then be added to the estimated resident population. Similarly, those travellers departing Australia (who are currently counted in the population) must be absent from Australia for a total of 12 months or more during the 16-month follow-up period to then be subtracted from the estimated resident population.

The 12/16 month rule does not have to be continuous and takes account of those persons who may have left Australia briefly and returned, while still being resident for 12 months out of 16. Similarly, it takes account of Australians who live most of the time overseas but periodically return to Australia for short periods.

Average annual growth rate

The average annual growth rate, \(r\), is calculated as a percentage using the formula:

\(\large{r=\left[(\frac{P_n}{P_o})^{\frac{1}{n}}-1\right] \times 100}\)

where \(\text{P}_0\) is the population at the start of the period, \(\text{P}_n\) is the population at the end of the period and \(n\) is the length of the period between \(\text{P}_0\) and \(\text{P}_n\) in years.


The delivery of a child, irrespective of the duration of pregnancy, who, after being born, breathes or shows any other evidence of life such as heartbeat.


A census is the complete enumeration of a specific population at a point in time (as opposed to a survey, which enumerates a sample of the population). When the word is capitalised, "Census" refers to the national Census of Population and Housing. The Census is run by the ABS every five years and aims to count every person in Australia on Census night.


The permanent disappearance of all evidence of life after birth has taken place. The definition excludes deaths prior to live birth.

For the purposes of the Deaths and Causes of Death collections compiled by the ABS, a death refers to any death which occurs in, or en route to Australia and is registered with a state or territory Registry of Births, Deaths and Marriages.

Estimated resident population (ERP)

The official measure of the population of Australia. It refers to all people, regardless of nationality, citizenship or legal status, who usually live in Australia, with the exception of foreign military or diplomatic personnel and their families. It includes usual residents who are overseas for less than 12 months over a 16-month period. It excludes overseas visitors who are in Australia for less than 12 months over a 16-month period.

Infant death

The death of a live-born child who dies before reaching their first birthday.

Infant mortality rate (IMR)

The number of infant deaths per 1,000 live births over the same period (typically year ended 30 June).

Intercensal difference

The difference between two estimates at 30 June of a Census year population: the first based on the latest Census, and the second arrived at by updating the 30 June estimate of the previous Census year with intercensal components of population change.

Interstate migration

The movement of people over a state or territory boundary for the purpose of changing their place of usual residence. Net interstate migration is the number of arrivals minus the number of departures.

Median age

The age at which half the population is older and half is younger.

Migration adjustment

Adjustments to overseas arrivals and departures data within the method used to estimate net overseas migration prior to September quarter 2006. These adjustments (previously referred to as category jumping) were mainly designed to reflect differences between stated travel intentions and actual travel behaviour.

Natural increase

The number of births minus the number of deaths.

Net overseas migration (NOM)

The net gain or loss of population through immigration to Australia and emigration from Australia.

Net undercount

The difference between the actual number of people who were counted in the Census (including imputations) and an estimate of the number of people who should have been counted in the Census. This estimate is based on the Post Enumeration Survey (PES) conducted after each Census. For a category of person (e.g. based on age, sex and state of usual residence), net undercount is the result of Census undercount, overcount, differences in classification between the PES and Census, and imputation error.

Overseas migrant arrivals

Incoming international travellers who stay in Australia for 12 months or more over a 16-month period, who are not currently counted within the population, and are then added to the population.

Overseas migrant departures

Outgoing international travellers who leave Australia for 12 months or more over a 16-month period, who are currently counted within the population, and are then subtracted from the population.

Population growth

Change in population over a particular period. Population growth is the sum of natural increase, net overseas migration and (for states and territories) net interstate migration. For periods prior to the latest Census, population growth also includes intercensal difference.

Population growth rate

Population change over a period as a proportion of the population at the beginning of the period, expressed as a percentage.

Post Enumeration Survey (PES)

The Census Post Enumeration Survey (PES) is a household survey conducted following the Census. The PES allows the ABS to estimate the number of people missed in the Census and the number counted more than once or in error. Historically more people are missed than are counted more than once in Australia, leading to a net undercount. PES estimates of net undercount are used to adjust Census counts for use in ERP.

Rebasing of population estimates

After each Census, the ABS uses Census counts (adjusted for undercount) to construct a new base population figure for 30 June of the Census year. Rebasing is the process of updating population estimates for the five years between Censuses, to incorporate information from the most recent Census.

Sex ratio

The number of males per 100 females in a particular population. For example, sex ratio at birth is the number of male births per 100 female births.

Standardised death rate (SDR)

The death rate (deaths per 1,000 or per 100,000) of a population, adjusted to remove the effects of the population’s age composition, allowing comparison of rates across populations with different age compositions. The standard population is available from data downloads – data cubes.

Standardised death rates enable comparison over time and between jurisdictions, by controlling for the impact of different age structures.

Total fertility rate (TFR)

The sum of age-specific fertility rates (live births at each age of mother per female population of that age) divided by 1,000. It represents the number of children a female would bear during her lifetime if she experienced current age-specific fertility rates at each age of her reproductive life (ages 15 - 49).

Usual residence

Within Australia, usual residence is the address of the dwelling at which a person considers themselves to currently live, either having lived there for some time or intending to live there for some time.

The usual residence of a newborn is that of the mother.

Quality declaration - summary

Institutional environment

Estimated Resident Population (ERP) uses data sourced from a variety of institutional environments. Much of the data is administrative by-product data collected by other organisations for purposes other than estimating the population. Births and deaths statistics are extracted from registers administered by the various State and Territory Registries of Births, Deaths and Marriages. Medicare Australia client address data and Defence data is used to estimate interstate migration. Information provided by the Department of Home Affairs from their Travel and Immigration Processing System is used to calculate overseas migration.

ABS Census of Population and Housing and Post Enumeration Survey (PES) data are used to determine a base population from which ERP is calculated and to finalise all components of population change. For information on the institutional environment of the Australian Bureau of Statistics, please see ABS Institutional Environment.


Estimates of the resident population (ERP) is the official measure of the population of states and territories of Australia according to a usual residence population concept. ERP is used for a range of key decisions such as resource and funding distribution and apportioning seats in the House of Representatives to each state and territory. ERP for the states and territories of Australia are published by sex and age groups, and estimates and projections of the Aboriginal and Torres Strait Islander population are also available.


Preliminary ERP data, disaggregated by sex and single year of age, is compiled and published quarterly and is generally made available six months after the end of each reference quarter.

Revised estimates are released once more accurate births, deaths and net overseas migration data becomes available. In the case of births and deaths, the revised data is compiled on a date of occurrence basis annually in the March reference period, released in September for the previous financial year.

In the case of overseas migration, final data is based on actual traveller behaviour. Final estimates for overseas migration are released quarterly and made available 18 months after the end of the reference period.

Final ERP estimates are made available every 5 years after a census when revisions are made to the previous intercensal period. ERP data is not changed once it has been finalised. Releasing preliminary, revised and final ERP involves a balance between timeliness and accuracy.


All ERP data sources are subject to non-sampling error. Non-sampling error can arise from inaccuracies in collecting, recording and processing the data. In the case of Census and PES data every effort is made to minimise reporting error by the careful design of questionnaires, intensive training and supervision of interviewers, and efficient data processing procedures. The ABS does not have control over any non-sampling error associated with births, deaths and migration data (see institutional environment).

Another dimension of non-sampling error in ERP is the fact that the measures of components of population growth become more accurate as more time elapses after the reference period. As discussed under Timeliness, the tradeoff between timeliness and accuracy means that a user can access more accurate data by using the revised or final ERP data. While the vast majority of births and deaths are registered promptly, a small proportion of registrations are delayed for months or even years. As a result, preliminary quarterly estimates can be an underestimate of the true number of births and deaths occurring in a reference period. Revised figures for a reference period incorporate births and deaths registrations that were received after the preliminary data collection phase as well as the estimated number of registrations that have still not been received for that reference period. For more information see the Demography Working Paper 1998/2 - Quarterly birth and death estimates (cat. no. 3114.0) and Population Estimates: Concepts, Sources and Methods (cat. no. 3228.0.55.001).

After each Census the ABS uses the Census population count to update the original series of published quarterly population estimates since the previous Census. For example, 2016 Census results were used to update quarterly population estimates between the 2011 and 2016 Census. The PES is conducted soon after the Census to estimate the number of Australians not included in the Census. Adding this net undercount of people back into the population is a crucial step in arriving at the most accurate ERP possible. For more information on rebasing see the feature article Rebasing of Australia's Population Estimates using the 2016 Census in the December quarter 2016 issue of Australian Demographic Statistics (cat. no. 3101.0). At the time of rebasing to the 2011 Census, estimates for the period September 1991 to June 2006 were revised in response to a methodological improvement in the estimation of Census undercount, for more information see Feature Article: Recasting 20 Years of ERP in Australian Demographic Statistics, December Quarter 2012 (cat. no. 3101.0).


ERP was introduced in 1981 and backdated to 1971 as Australia's official measure of population based on place of usual residence. ERP is derived from usual residence census counts, to which is added the estimated net census undercount and Australian residents temporarily overseas at the time of the census (overseas visitors in Australia are excluded from this calculation). Before the introduction of ERP, the Australian population was based on unadjusted census counts on actual location basis. It is important to note this break in time series when comparing historical population estimates.

An improved definition for calculating overseas migration was applied from September quarter 2006 onwards. The key change is the introduction of a '12/16 month rule' for measuring a person's residency in Australia replacing the '12/12 month rule'. This change results in a break in time series and therefore it is not advised that overseas migration data calculated using the new method is compared to data previous to this. For further information see Information Paper: Improving Net Overseas Migration Estimation (cat. no. 3412.0.55.001).

The births and deaths data in this publication are not coherent with the data found in ABS births and deaths publications. This is because the revision cycle necessary to produce ERP results in a mix of preliminary births and deaths data, based on date of registration, and revised data which is a modelled estimate of births and deaths by date of occurrence. By contrast, the main tables of data in the births and deaths publications are based wholly on registration in the reference year, with some tables and analysis based wholly on date of occurrence data.


ERP is generally easy to interpret as the official measure of Australia's population (by state and territory) on a place of usual residence basis. However, there are still some common misconceptions. For example, a population estimate uses the term 'estimate' in a different sense than is commonly used. Generally the word estimate is used to describe a guess, or approximation. Demographers mean that they apply the demographic balancing equation by adding births, subtracting deaths and adding the net of overseas and interstate migration to a base population. Each of the components of ERP is subject to error, but ERP itself is not in any way a guess. It is what the population would be if the components are measured well.

Population estimation is also very different to sample survey-based estimation. This is because population estimation is largely based on a full enumeration of components. In the case of the population base, only the PES used sampled data to adjust for census net undercount. In the case of the components of population growth used to carry population estimates forward, Australia has a theoretically complete measure of each component.

Another example of a common misconception relates to the fact that the population projections presented in this publication are not predictions or forecasts. They are an assessment of what would happen to Australia's population if the assumed levels of components of population change - births, deaths and migration - were to hold into the future.


ERP data is available in a variety of formats on the ABS website under the population product family. The formats available free on the web are:

  • The main features which has the key figures commentary,
  • Time series spreadsheets on population change, components of change and interstate arrivals and departures,
  • Spreadsheets containing the key data tables,
  • ABS.Stat datasets with population and the components of change, which can be extracted in various formats

If the information you require is not available as a standard product, then ABS Consultancy Services can help you with customised services to suit your needs. For inquiries contact the National Information and Referral Service on 1300 135 070. Alternatively, please email


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ABSAustralian Bureau of Statistics
ACTAustralian Capital Territory
ASGSAustralian Statistical Geography Standard
ERPestimated resident population
IMRinfant mortality rate
NIMnet interstate migration
NOMnet overseas migration
NSWNew South Wales
NTNorthern Territory
OADoverseas arrivals and departures
PESCensus Post Enumeration Survey
SASouth Australia
SDRstandardised death rate
TFRtotal fertility rate
WAWestern Australia
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