National, state and territory population methodology

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Reference period
June 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-Jun. 2020RevisedFinalPreliminaryREVISED 
Sep. 2020 - Mar. 2021Preliminary RevisedPreliminaryPRELIMINARY - updated due to revised component data 
Jun. 2021Preliminary PreliminaryPreliminaryPRELIMINARY

Rebasing method

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

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

(2) adding Australian usual residents temporarily overseas on Census night

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

This new ERP becomes the base for quarterly estimates 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

June 2021 and 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).

Current issues

June 2021: Medicare change of address data showed an implausibly high number of moves for this quarter due to widespread updating of Medicare records as people get vaccinated for COVID-19. Not all the address changes recorded in this quarter happened within this quarter. To treat for this, under-count adjustments in this quarter and those made in previous quarters have been revised.  Interstate migration was revised for September 2020, December 2020 and March 2021 quarters in this release. 


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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