ESTIMATED RESIDENT POPULATION AND THE REBASING CYCLE

BACKGROUND

The official estimate of Australia's population is known as the estimated resident population (ERP). It is published by the ABS each quarter at the national, state and territory level, and annually for sub-state regions.

ERP is used to inform evidence-based decisions such as:

• the allocation of seats to states and territories in the House of Representatives of the Australian Parliament
• the distribution of Commonwealth payments to states and territories
• demographic, social and economic research
• policy research, evaluation and government reporting.

The ABS undertakes a Census of Population and Housing (the Census) every five years as required by the Census and Statistics Act 1905. The Census is used to prepare an ERP base, which is then used to prepare estimates for the following five years, until the next Census when the process is repeated. A regular Census enables the updating and maintenance of an accurate time series of Australia's population estimates and limits the accumulation of errors to a five-year period. This process is called 'rebasing' and as part of this process, ERP is revised (rebased) back to the previous Census.


OVERVIEW OF THE REBASING PROCESS

The rebased 30 June ERP is constructed from the Census count in two main steps. Firstly, the usual resident population as at Census night is estimated by adjusting for Census net undercount as derived from the Post Enumeration Survey (PES), applying any demographic adjustments, and adding Australian residents temporarily overseas (RTOs). The ERP at Census night is then backdated to 30 June by accounting for the population change components (births, deaths and migration) that took place over this time.

For national, state and territory ERP, these steps are represented in the following diagram:

Once the rebased ERP has been prepared from the new Census, estimates back to (but not including) 30 June of the previous Census year are revised.


UPDATING THE ESTIMATED RESIDENT POPULATION

ERP is maintained between Censuses through an accounting framework that updates the base ('stock') population with data on the 'flow' of population change components (births, deaths and migration). The main data sources for these components are birth and death registrations, passenger information and visa movements for overseas migration, and Medicare data for internal migration.

The accounting framework is represented by the demographic balancing equation:

P_t = P_o + B – D + M

where:
P_t is the population being estimated
P_o is the base population
B is births over the period
D is deaths over the period
M is net migration over the period.


PRELIMINARY AND FINAL REBASED ESTIMATES

Preliminary rebased ERP is released as soon as possible after the Census and before 30 June of the year following the Census to meet legislative reporting requirements. Preliminary rebased ERP is updated to final around 12 months later when final net overseas migration data becomes available for use in both the RTO calculation and for backdating. Final rebasing also allows updated births, deaths and internal migration (from the latest Census) to be incorporated into the backdating step.

Preliminary rebased ERP from the 2016 Census at national, state and territory level was released on 27 June 2017, and at sub-state level on 28 July 2017. Final rebased ERP from the 2016 Census will be released in mid-2018.

For further information, refer to the Feature Article: 'Rebasing of Australia Population Estimates Using the 2016 Census' in Australian Demographic Statistics, Dec 2016 (cat. no. 3101.0).