Regional population 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.

Annual population estimates as at 30 June are then prepared for areas below the state level 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 (ASGS), and are aggregated or split to create estimates for other geographies. Population estimates are available in this product for Statistical Areas Levels 2 to 4, Greater Capital City Statistical Areas, Local Government Areas, Significant Urban Areas, Remoteness Areas, and Commonwealth and State Electoral Divisions. This is the first release of Regional population to include data on the Australian Statistical Geography Standard (ASGS) Edition 3.

The estimates initially released in this issue on 26 July 2022 were preliminary rebased for 2017 to 2021, based on the 2021 Census. The commentary contained in this issue is based on these estimates. Age and sex breakdowns of these preliminary rebased totals were released on 30 August 2022 in Regional population by age and sex, 2021.

Final 2021 Census-based estimates for 2017 to 2021 were added to the Data downloads section of this issue on 31 August 2023. Age and sex breakdowns of these final rebased totals were added to the Data downloads section of Regional population by age and sex, 2021 on the same date. Final rebased estimates supersede all previously released data. 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 information on the demographic adjustments made for national and state/territory ERP, see Methodology used in rebased population estimates, June 2021.

At the greater capital city and rest of state level (part of state), a further demographic adjustment was applied to account for relatively large variations in the PES net undercount adjustments between Censuses.

The following two populations were used to arrive at an adjusted 2021 rebased ERP for each part of state:

• the unadjusted 2021 rebased ERP for that part of state, and
• the 2021 unrebased ERP for that part of state (as published in Regional population, 2020-21)

A weighting of 60% was applied to the unadjusted 2021 rebased ERP split of population between the greater capital city and rest of state region for each state/territory, and 40% to the 2021 unrebased ERP split. Within each state/territory, the population for each part of state was then adjusted to arrive at this composite split (with balancing adjustments made so that the net adjustment at the state/territory level was zero), as shown in the following table.

ERP for all regions below each part of state was then apportioned to the new part of state ERP on a pro-rata basis. This composite method was also applied for 2016 rebasing of sub-state ERP.

For more information, including the differences between preliminary and final rebased estimates, see Methodology used in final rebased population estimates, June 2021.

In Census years, both preliminary estimates (derived from updating ERP from the previous Census) and 'rebased' population estimates (based on the current Census) are prepared. Differences between these two sets of estimates are referred to as intercensal differences. Rebased estimates of SA2 populations for previous intercensal years are derived by apportioning the intercensal difference across the five years, while constraining to state totals. Rebased 2017 to 2020 estimates were generally derived by adding one-fifth of the 2021 intercensal difference to the previous estimate of the 2017 population, two-fifths to the previous estimate of the 2018 population, and so on. Intercensal difference was apportioned based on the unrebased growth rate for some areas (e.g. newly established areas).

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

The sub-state estimates in this product are subject to some error. Some caution should be exercised when using the estimates, especially for areas with very small population.

An indication of the accuracy of ERP can be gauged by assessing the size and direction of intercensal differences. For Australia as at 30 June 2021, the unrebased ERP over-estimated the final rebased ERP by 0.2% (62,400 people). For the states and territories, the 2021 intercensal differences ranged from -3.7% (Tasmania) to +2.1% (Northern Territory).

To assess the quality of SA2-based estimates, unrebased estimates for 2021 were converted to SA2s based ASGS Edition 3 boundaries, and constrained to final rebased state/territory ERP. These estimates were compared with final rebased 2021 SA2 estimates. The average of the absolute values of the intercensal differences for these SA2 estimates (excluding areas with less than 1,000 people) was 4.2%.

The table below shows that intercensal differences were generally larger for very small areas, and lower for very large areas.

To meet the competing demands for accuracy and timeliness, there are several versions of sub-state population estimates. Preliminary estimates are available around nine 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. Estimates in this issue are final for 2001 to 2021. 

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 and deaths 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 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 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). 

Medicare is Australia's universal health insurance scheme and covers the vast majority of Australian residents. De-identified Medicare change of address counts 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 moving house 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 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 as published in National, state and territory population.

RIME was previously prepared and released in Migration, Australia for financial years up to 2015-16. This old series of RIME was experimental in that it was prepared independently of and is not directly comparable with ERP nor with RIME prepared for 2016-17 onwards, due to different methods and source data used. The old RIME series used Medicare change of address data aggregated to postcodes, which was converted to SA2/LGA, and was supplied to the ABS quarterly meaning that one person could record up to four moves in a financial year. RIME for 2016-17 onwards uses change of address data coded directly to ASGS areas, and is supplied and calculated on an annual basis consistent with the definition of population change over a financial year.

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 contains data for the ABS Structures of both the 2021 ASGS (Edition 3) and 2016 ASGS (Edition 2) and is the last release of population estimates on the 2016 version. Further information on the ABS Structures of the 2021 ASGS (Edition 3) 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 for Statistical Areas Level 2 to 4 and Greater Capital City Statistical Areas

Further information on the ABS Structures of the 2016 ASGS (Edition 2) for which population estimates are available in this product is contained in:

• Australian Statistical Geography Standard: Volume 1 - Main Structure and Greater Capital City Statistical Areas, July 2016 for Statistical Areas Level 2 to 4 and Greater Capital City Statistical Areas
• Australian Statistical Geography Standard: Volume 4 - Significant Urban Areas, Urban Centres and Localities, Sections of State, July 2016 for Significant Urban Areas 
• Australian Statistical Geography Standard: Volume 5 - Remoteness Structure, July 2016 for Remoteness Areas 

The Non-ABS Structures 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. Further information on the Non-ABS Structures of the 2021 ASGS (Edition 3) for which population estimates are available in this product is contained in:

• Australian Statistical Geography Standard (ASGS) Edition 3 - Non ABS Structures, June 2021 for Local Government Areas, Commonwealth Electoral Divisions and State Electoral Divisions

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

The area figures used in this product were calculated using ABS standard Geographic Information System software from the digital boundaries of the ASGS. 

The centre of population of a region is a point that describes a centre point of the region's population, and is calculated in this product based on SA1s. Due to the inherent imprecision in small area estimates, the centre of population should be considered indicative and not ascribed to an exact location. The use of different geographical level data in the calculation of the centre of population can result in different locations.

The population density of each region in this product has been calculated by dividing its ERP by its area in square kilometres. The result is expressed as a number of people per square kilometre.

In this product, ERP is also presented in one square kilometre grid format. The population grid offers a consistently sized spatial unit and gives a refined model of population distribution, particularly for the non-urban areas of Australia. It is also an established, easy to understand and readily comparable international standard which enables users to make local, national and international comparisons of population density.

The population grid is prepared using SA1 population estimates. Within each populated SA1, all known residential dwelling locations were identified using a subset of the ABS Address Register, and the population distributed equally across the residential dwellings. The average value assigned to each dwelling was then summed within each one square kilometre grid cell across the country. This is modelled data and caution must be used in its interpretation, as the population has not been measured at the one square kilometre grid cell level.

Prior to 2021, the grid was prepared using known residential dwelling locations based on the Geocoded National Address File. The new methodology used for 2021, which makes use of the ABS Address Register, has resulted in the population grid showing a more targeted representation of the population.

The population grid is provided in ESRI Grid format and Geo TIFF format, which are recommended for users proficient in the use of Geographic Information System software.

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, a procedure is applied to confidentialise sub-state ERP and components, which are also subsequently constrained so that they add to relevant state estimates. As a result of this confidentialisation method, and forced additivity, estimates 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.