Methodology used in final rebased population estimates, June 2021

Methods and quality assurance of final rebased estimated resident population (ERP) at 30 June 2021 and ERP revision back to 30 September 2016


Final rebased ERP

September 2016 to June 2021 final rebased estimates supersede all previously released estimates.

Final rebasing is the final revision of the estimated resident population (ERP) based on the latest Census.

We have now finalised population estimates for September 2016 - June 2021 using the results of the 2021 Census of Population and Housing and more up-to-date component data.

Final rebased estimates supersede all previously released estimates.

The preliminary rebased population estimates were published in National, state and territory population, December 2021. Preliminary rebasing is described in Methodology used in rebased population estimates, June 2021.

Final population estimates are first published in National, state and territory population, December 2022 (release date 15/06/2023).

Estimation of the final base population

We calculate the final rebased ERP at 30 June in a Census year by:

  • adjusting Census counts of Australian usual residents to account for people missed or counted twice in the Census (based on Post Enumeration Survey results)
  • demographically adjusting Census counts or net undercount estimates based on coherence with other data sources and to remove implausible demographic features
  • adding usual residents temporarily overseas on Census night
  • adjusting for births, deaths and migration from 30 June to Census night.

This final rebased ERP becomes the base for quarterly estimates going forward from that point until the next rebasing.

The steps are illustrated in the following diagram.

Diagram of the estimated resident population rebasing process

Diagram of the estimated resident population rebasing process

Diagram of the estimated resident population rebasing process

The diagram shows how different elements of the ERP rebasing process are connected. The process is in two parts and begins with the Census numbers for persons at place of usual residence at census night. The first part of the process starts with adding in persons who were missed by the Census by using a number of estimates. PES net undercount estimates are included first. Then, Australian residents who are temporarily overseas are added in. Demographic confrontation with other sources of population datasets also informs how many persons were missed. This first process gives us the ERP for Australia at Census night. The second part of the process is about taking the ERP at Census night back to 30 June 2021. Births that have occurred in between this period are removed. Deaths that have occurred in the same period are added back into the population count. Persons who have moved between states and territories from 30 June to Census night are moved back. Persons who have moved into or, out of, Australia during this period are also moved back. That is how we arrive at ERP for 30 June 2021.

Adjustment components of ERP, Final, 30 June 2021
 Persons ('000)
 Census count, place of usual residence8,072.26,503.55,156.11,781.52,660.0557.6232.6454.525,422.8
Including people who have been missed
 Plus – net undercount-2.620.950.
 Plus – demographic adjustment13.
 Plus – residents temporarily overseas13.611.
Equals – ERP at Census night8,096.16,546.45,221.51,803.02,751.9567.7248.5453.325,693.3
Backdating to 30 June 2021
 Minus – births11.
 Plus – deaths7.
 Minus – net interstate migration-2.4-
 Minus – net overseas migration-2.8-2.2-0.7-0.2-
Equals – ERP at 30 June 20218,097.16,547.85,215.81,802.62,749.4567.2248.2452.525,685.4

- nil or rounded to zero (including null cells)
a. Includes Other Territories.

Additional information

For more information about the 2021 Census see Population: Census, 2021.

For more information about the Census Post Enumeration Survey see 2021 Census overcount and undercount.

Comparison to preliminary rebasing

This process is similar to that followed to produce preliminary rebased ERP (published in June 2022). Final rebasing is primarily a process of revising preliminary rebased data incorporating revisions to;

  • estimates of Residents Temporarily Overseas, and
  • components of population growth (births, deaths and migration) used to backdate from Census night to 30 June
Difference between preliminary and final estimated resident population, 30 June 2021
New South Wales8,097.18,
South Australia1,802.61,803.2-0.6-0.03
Western Australia2,749.42,749.9-0.5-0.02
Northern Territory248.2249.2-1.0-0.42
Australian Capital Territory452.5453.6-1.1-0.23

- nil or rounded to zero (including null cells)
a. Includes Other Territories

Residents temporarily overseas

The preliminary residents temporarily overseas (RTO) estimates used the behaviour of travellers at the same time the previous year to predict the behaviour of people who were overseas on Census night. Final RTO estimates are based on actual traveller behaviour.

Backdating components

Preliminary rebasing used preliminary estimates of births, deaths and migration for the period 1 July - 10 August 2021. Components for this period have since been finalised, and the backdating recalculated to incorporate the updated component estimates.

Demographic adjustment

For preliminary rebasing, some demographic adjustments were made which addressed these issues:

  • age misreporting
  • unreliable estimation of the centenarian population
  • implausible sex ratios
  • implausible features due to sampling error in the Census Post Enumeration Survey (PES).

For final rebasing, no further demographic adjustment was made.

Review of intercensal components of growth

The final rebasing process allows us an opportunity to review estimates of births, deaths, interstate migration and overseas migration estimates from September 2016 to June 2021. Revisions may be made if there is more current data available.

For final rebasing, interstate migration estimates have been revised based on updated data from Census.

Births, deaths and overseas migration estimates were not revised.

Revision to interstate migration data

At final rebasing we revise the modelled interstate migration estimates for the intercensal period with data from Census questions related to an individual's place of usual residence one year ago and at Census night. This process and its outcome is detailed in 2021 Census update of the Net Interstate Migration model.   

Revision to overseas migration data

There was no revision to the overseas migration estimates.  This is because there is no additional data source on overseas migration for the intercensal period that suggested a revision was warranted since preliminary rebasing.

Intercensal difference

Final intercensal difference is the difference between the final Census-based estimate (known as rebased ERP) and the finalised quarterly component-based estimate (known as unrebased ERP). We calculate it by state and territory, sex and age.

Intercensal difference may result from:

  • error in the 2016 Census based population estimate
  • error in the 2021 Census based population estimate and/or
  • error in the quarterly estimates of births, deaths, interstate and overseas migration.

For final rebasing, we are able to reduce the preliminary intercensal difference by explaining some of the population change in terms of demographic components eg finalising interstate migration.

After final rebasing is complete, it is generally not possible to attribute intercensal difference to a particular source, or to quantify the error that each source contributes. 

To calculate the final rebased population estimates from September quarter 2016 to 30 June 2021, we have revised quarterly population estimates between the current and previous Censuses by assuming intercensal difference accumulated evenly across the five-year period.

Between preliminary rebasing and final rebasing, intercensal difference improved for all states and territories apart from Queensland and Northern Territory.

For the 2016–2021 period, the final intercensal difference for Australia was 62,404 compared with a preliminary intercensal difference of 52,211. This change was due to revisions in components including Net Overseas Migration (NOM) backdating and RTO's as well as revisions to unrebased ERP. In percentage terms, the national intercensal difference stayed the same at 0.2%

Preliminary and final intercensal difference(a), 2016-2021
  Preliminary Final

a. Negative numbers mean that the unrebased ERP underestimated the population. Positive numbers means that the unrebased ERP was an overestimate.
b. Includes Other Territories.

Final intercensal difference by five year age groups(a), 2016-2021
 Intercensal difference 
Age group (years)'000%
85 and over13.72.49
All age groups62.40.24

a. Negative numbers mean that the unrebased ERP underestimated the population. Positive numbers means that the unrebased ERP was an overestimate.

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