Regional population by age and sex methodology

Estimated resident population (ERP) is the official estimate of the Australian population, which links people to a place of usual residence within Australia. Usual residence is the address at which a person considers themselves to currently live. ERP includes all people who usually live in Australia (regardless of nationality, citizenship or legal status), with the exception of foreign diplomatic personnel and their families.

ERP, or population estimates, for Australia and it's states and territories (from now on referred to as states) are prepared quarterly and released around six months after the reference date in National, state and territory population.

Population estimates for areas below the state level (from now on referred to as sub-state) are then prepared annually. Population totals for sub-state areas are released around nine months after the 30 June reference date in Regional population. Age and sex breakdowns of these estimates are then prepared and released in this product. Estimates are prepared at the Statistical Area Level 2 and Local Government Area levels, according to the Australian Statistical Geography Standard, and are aggregated or split to create estimates for other geographies. Population estimates for Statistical Areas Levels 2 to 4, Greater Capital City Statistical Areas and Local Government Areas are available in this product.

The estimates initially released in this issue on 30 August 2022 were preliminary rebased for 2017 to 2021, based on the 2021 Census. The commentary contained in this issue is based on these estimates.

Final 2021 Census-based estimates for 2017 to 2021 were added to the Data downloads section of this issue on 31 August 2023. These estimates supersede all previously released estimates, and are consistent with the totals released on 31 August 2023 in Regional population, 2021 and the state and territory estimates by age and sex released on 15 June 2023 in National, state and territory population, December 2022. No commentary has been prepared based on final rebased estimates. 

The 2021 estimates in this release are based on the 2021 Census, with adjustments made for residents temporarily overseas, net Census undercount and overcount, and the time between Census night and 30 June. These are referred to as rebased population estimates. For more information, including the differences between preliminary and final rebased estimates, see Methodology used in final rebased population estimates, June 2021.

After each Census, the ABS uses Census counts 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. Rebased population estimates by age and sex for each sub-state area incorporate the previous and following Census year estimates, by age cohort and sex. They are constrained to a rebased total population for each intercensal year.

As a result of the rebasing process, the components by age and sex (natural increase, internal and overseas migration) published in previous issues of this product no longer sum to population change by age and sex.

Component data (births, deaths and overseas and internal migration) are confidentialised and constrained to add to the relevant state component estimates by age and sex. The resultant ERPs are subsequently constrained to state population estimates by age and sex. As a result of confidentialisation and forced additivity, estimates of under three people should be regarded as synthetic and only exist to ensure additivity to higher levels. While output is presented by five-year age group, all calculations are made at single year of age level. Population estimates based on single year of age may be available on request via the ABS website. 

While sub-state ERP by age and sex is consistent with state ERP by age and sex, small differences arise between aggregated sub-state components by age and otherwise published state components by age. This is due to the different reference period of sub-state ERP (annual as at 30 June) and state ERP (quarterly), as the age of a person at the end of the financial year can be different to the age of that person at the end of each quarter.

The estimates in this product are subject to some error. Some caution should be exercised when using the estimates, especially for areas with very small populations. Areas with a total population of less than 1,000 people are excluded from commentary.

To meet the competing demands for accuracy and timeliness, there are several versions of sub-state population estimates. Preliminary estimates by age and sex are available around 14 months after the reference date with revised estimates 12 months later. Rebased and final estimates are made available after each Census, when revisions are made to the estimates for all years in the previous intercensal period.

The status of annual sub-state ERP and components changes over time, from preliminary to revised to final, as new component data becomes available at the state level. The estimates in this product are final for 2001 to 2021.

More information on the method, accuracy and status of these estimates is contained in Regional population, methodology. 

Natural increase (births minus deaths) for sub-state areas is calculated using information provided by each state/territory registry of births, deaths and marriages. The data is coded based on the place of usual residence of the mother for births, and the place of usual residence of the deceased for deaths. It is aggregated to SA2 and LGA levels and constrained to published state estimates of births and deaths.

The estimates of births (by sex) and deaths (by age and sex) in this product are prepared for financial years to correspond with the 30 June reference date for sub-state ERP. To produce timely sub-state estimates, preliminary births and deaths data are prepared using year of registration as a proxy for year of occurrence.

Preliminary births and deaths are prepared by breaking down preliminary state-level data. Later, when the state-level data is updated, the sub-state data is updated accordingly and released in the next issue of this product. 

The sub-state births and deaths data in this product is not coherent with the sub-state data released in Births, Australia and Deaths, Australia which is for calendar years and has a different scope.

The movement of people from overseas to Australia's sub-state areas and vice-versa cannot be directly measured and is estimated by breaking down overseas migrant arrivals and departures, by age and sex, at the state level to sub-state areas using information from the most recent Census. The state-level overseas migration data is sourced from Department of Home Affairs processing systems, visa information, and incoming passenger cards, and is published in National, state and territory population.

Regional overseas migration estimate (ROME) arrivals are estimated based on counts of people who identified in the Census that they were living overseas one year ago, at SA2 level. This distribution is used to break down state arrivals each year up until the next Census. To account for changes to the distribution of overseas arrivals within a state between Censuses (e.g. in high growth areas or inner-city areas with changing numbers of temporary migrants), adjustments may be made based on up-to-date indicator data including counts of Temporary Skills Shortage visa holders and overseas students. 

For ROME departures, a model distributes state-level overseas migrant departures to SA2s. This model is based on a range of information from the Census - mainly the number of people who arrived in each area from overseas in the last year. More weighting is given to areas that have high SEIFA Index of Education and Occupation scores and more than 20% of their total population born overseas. Of all the models evaluated, this model was selected as it best estimated population change between the last two Censuses. As with overseas arrivals, overseas departures may be adjusted based on additional information sources.

LGA estimates of ROME arrivals and departures by age and sex are prepared by converting from SA2-level ROME arrivals and departures, using a population-weighted correspondence.

Preliminary ROME arrivals and departures are prepared by breaking down preliminary state-level data. Later, when the state-level data is updated, the sub-state data is updated accordingly and released in the next issue of this product.

The movement of people between and within Australia's states and territories cannot be directly measured and is estimated using administrative data. Internal 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), by age and sex. 

Medicare is Australia's universal health insurance scheme and covers the vast majority of Australian residents. De-identified Medicare change of address counts by age and sex are aggregated to SA2 and LGA levels. 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 this data to internal migration data from the Census. These factors are applied by age, sex, state and move type (arrival or departure). Medicare data received for the year ending 30 September is used to estimate internal migration for the year ending 30 June. This assumes that on average the time between a person changing residence and registering their change of address with Medicare is three months. 

As many defence force personnel do not interact with Medicare, defence movements data is used to supplement the Medicare data. Aggregated defence force personnel movements by age and sex are converted from postcode to SA2 and LGA levels. This data reflects the time of move, and is therefore not lagged.

The Medicare and defence data are combined to prepare regional internal migration estimates (RIME) at SA2 and LGA levels. Interstate RIME moves are constrained to estimates of interstate migration by age and sex as published in National, state and territory population.

The Australian Statistical Geography Standard (ASGS) brings together all of the regions which the ABS and many other organisations use to collect, release and analyse geographically classified statistics. The ASGS classification structures are split into two broad groups, ABS Structures and Non-ABS Structures.

The ABS Structures are defined and maintained by the ABS, and remain unchanged for the five years between Censuses. This issue of Regional population by age and sex contains data for the ABS Structures of both the 2021 ASGS (Edition 3) and 2016 ASGS (Edition 2). Further information on the ABS Structures for which population estimates are available in this product is contained in:

• Australian Statistical Geography Standard (ASGS) Edition 3 - Main Structure and Greater Capital City Statistical Areas, July 2021 - June 2026 
• Australian Statistical Geography Standard: Volume 1 - Main Structure and Greater Capital City Statistical Areas, July 2016

The Non-ABS Structures, including Local Government Areas (LGAs), are not defined or maintained by the ABS, and generally represent administrative regions. As the Non-ABS Structures represent regions that are subject to ongoing change, the ABS releases updates to these structures each year where significant change has occurred. When boundaries for Non-ABS Structures such as LGAs change, historical population estimates for these new boundaries are prepared to enable the comparison of populations over time. The population estimates available in this product are based on 2021 Local Government Area boundaries. For further information, see Australian Statistical Geography Standard (ASGS) Edition 3 - Local Government Areas, July 2021 - June 2026.

Maps of the statistical areas defined in the ASGS are available in the online mapping tool ABS Maps.

The population of the Other Territories, namely Christmas Island, Cocos (Keeling) Islands, Jervis Bay and Norfolk Island, is included in all references to the total population of Australia. However, the Other Territories are excluded from the commentary in this product.

Population pyramids illustrate the age and sex distribution of a population. In this product, five-year age groups are represented on the vertical axis of the graph. The population in each age-sex group is expressed as a percentage of the total population in that area. Thus, the sum of the percentages indicated by the darker bars is 100% of the population of the area represented by the darker bars, e.g. Greater Sydney. Further, the sum of the percentages indicated by the lighter bars is 100% of the area represented by the lighter bars, e.g. the rest of NSW. By using this method, the age and sex distribution of two areas can be compared irrespective of the relative sizes of the total populations of the areas.

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, all small data cells in this product are confidentialised. Any cells of under three people should be regarded as synthetic and only exist to ensure additivity to higher levels.

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 by 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.