WA Tomorrow 13 - Methodology

Cohort component model

The WA Tomorrow 13 State forecast uses a Cohort Component Model (CCM) to produce population forecasts by age and sex for Western Australia. The CCM applies assumptions about future fertility, mortality and migration to a base population. These assumptions are informed by analysis of historical and recent demographic trends.

The model uses the following demographic equation:

Future population = base population + births − deaths + in-migration − out-migration

The model produces forecasts of the total population, its age-sex structure, and the future numbers of births, deaths and migrants.

Base population

The WA Tomorrow 13 State population forecast is based on the Australian Bureau of Statistics’ (ABS) Estimated Resident Population (ERP) as at 30 June 2025, published in June 2026. The 2025 ABS ERP estimates are preliminary and based on the 2021 Census of Population and Housing. The ERP for 2022 to 2026 will be finalised in 2028 following the 2026 Census rebasing process.

The ERP is the official measure of Australia’s population and includes all people who usually live in Australia, except for foreign diplomats. The WA Tomorrow forecasts refer to the population as at 30 June for each stated year, consistent with the ABS’ ERP. For more information on the ERP, visit ABS: National, state and territory population.

The WA Tomorrow 13 State population forecast assumptions are informed by detailed analysis of demographic data.

Fertility

Fertility assumptions are informed by ABS historical births data, by year of occurrence and registration, including data on the age of the mother. This is complemented by more recent WA Department of Health births data.

The WA Tomorrow model applies age-specific fertility rates (ASFR) and sex ratio at birth for each projection year to estimate the number of babies born. The ASFRs represent the number of births per 1,000 women in each age group each year. The WA Tomorrow 13 State forecast assumes relatively stable ASFRs across the forecast period.

The Total Fertility Rate (TFR) summarises ASFR into the average number of children a woman would give birth to during her lifetime if current age-specific fertility rates continued. The TFR is assumed to be 1.44 births per woman in 2026 and to remain constant thereafter. The TFR has declined in recent years, partly reflecting migration-driven growth in the number of women of childbearing age. Despite the lower TFR, the number of births has increased slightly in recent years and is forecast to continue rising over the projection period.

For all forecast years, the sex ratio at birth is assumed to be 105.5 male births per 100 female births, based on current data.

Mortality

Mortality assumptions are based on ABS historical deaths data, including age at time of death.
The WA Tomorrow model applies age-specific mortality rates (ASMR) to estimate the number of deaths in each forecast year. Mortality rates are assumed to continue improving over the forecast period for people aged up to 100 years, with female life expectancy remaining higher than male life expectancy.
Despite improving mortality rates, population ageing is forecast to result in an increase in both the total number of deaths and the crude death rate. The crude death rate is the number of deaths occurring during a year per 1,000 people. It is forecast to increase from 6.0 deaths per 1,000 people in 2025 to 7.46 in 2046.

Migration

Future assumed levels of Net Overseas Migration (NOM) and Net Interstate Migration (NIM) are informed by ABS historical data on migration, overseas arrivals and departures by visa category and Australian Government forecasts. Assumptions were developed in consultation with the WA Department of Treasury and Finance.

The WA Tomorrow model applies assumptions on future levels of NOM and NIM, along with the age-sex profiles of overseas and interstate arrivals and departures.

NOM is the most volatile component of Western Australia’s population change, reflecting changes in economic cycles and employment opportunities. Although rates vary considerably, the state has received a larger share of national net overseas migration has generally been higher than its share of Australia’s population over the past four decades.

NOM volatility increased with pandemic-era border closures and subsequent strong growth levels, particularly for overseas students and working holiday makers.

After peaking in 2022–23, NOM is projected to return to levels closer aligned to the long-term average from 2026-27, as migration flows normalise. WA Tomorrow modelling does not incorporate any assumptions regarding proposed changes to immigration policy settings.

NIM levels currently remain elevated. Historically NIM has fluctuated with Western Australia experiencing different periods with net interstate inflows or outflows. On average, there has been net interstate inflows of around 2,670 people per annum. NIM is expected to remain elevated in 2026–27, before steadily returning towards its long-term average from 2028–29 onwards.

Geography

The WA Tomorrow 13 sub-state population forecasts have been produced for two geographies:

• Statistical Area Level 2 (SA2); and
• Local Government Area (LGA).

The sum of all respective sub-state forecasts equals the WA Tomorrow 13 State forecast for the central band.

The SA2 boundaries are based on the ABS 2021 Australian Statistical Geography Standard (ASGS, Edition 3). For WA Tomorrow 13, SA2s that contain little or no population have been amalgamated with adjacent SA2s (for example, industrial or undeveloped areas). 

Western Australia has a relatively large number of LGAs, including across sparsely populated regional areas. LGA populations have been derived via a geographical and population concordance of SA2 forecasts.

SA2 total population forecast

Total population forecasts for each SA2 are a key input into WA Tomorrow 13. These forecasts are derived through extrapolating historical SA2 population trends. Across Greater Perth and major regional centres, SA2 forecasts are constrained by estimated future dwelling capacity. Future dwelling capacity is estimated through a tiered spatial analysis of planning datasets, including existing dwellings, residential density coding of individual lots, and anticipated development across activity centres, structure plans and future greenfield areas.

SA2 age and sex forecast method

WA Tomorrow 13 is an intercensal update and uses a bespoke method to produce SA2 age and sex forecasts. The age-sex distributions from the WA Tomorrow 12 SA2 forecasts, which were produced using a sub-state CCM and region-specific demographic assumptions, provide the initial population structure.

Iterative Proportional Fitting (IPF) is then used to adjust these distributions to align with both the WA Tomorrow 13 State age-sex forecast and the SA2 total population forecasts. This approach ensures consistency across geographic scales while retaining the underlying demographic patterns from the WA Tomorrow 12 SA2 forecasts.

Local Government Area age and sex forecast method

Local Government Area population forecasts are derived from the SA2 forecasts using a population concordance (or apportionment) approach. The SA2 population forecasts are disaggregated to the SA1 level by age and sex and then re-aggregated to LGA level.
The concordance proportions for this approach are calculated based on ABS ERP data at SA1 level, by age and sex, as well as the total population.

Understanding the bands

It is common practice to prepare a population forecast range to reflect the inherent uncertainty in projecting future population outcomes. WA Tomorrow 13 provides five forecast bands to represent this uncertainty, with the same band percentiles used for the state and sub-state forecasts. These bands provide a structured range of lower, central and higher population forecast outcomes.
Band C is the central, or median, forecast, with an equal chance that the actual population will be higher or lower. Bands A to E show progressively higher population outcomes and associated respective probabilities.

Bands A to E represent 90% of the forecast range, while Bands B to D represent the central 50% range.

For some purposes, considering the full forecast range or specific forecast bands may be more appropriate than relying solely on the central forecast.

WA Tomorrow 13 has applied different techniques to generate a forecast range for the state and sub-state forecasts.

State forecast uncertainty

The WA Tomorrow 13 State forecast uses a probabilistic method to produce the forecast range. A deterministic model based on a single set of assumptions is first used to produce the central forecast, Band C. Uncertainty around the central forecast is then estimated by running 100,000 model simulations. The distribution of simulated population outcomes is used to derive the 5th, 25th, 50th, 75th and 95th percentile forecasts represented by Bands A to E.

Sub-state forecast uncertainty

The WA Tomorrow 13 sub-state population bands at SA2 level are produced using a residual bootstrap, or resampling, percentile projection methodology. Historical variation in population growth is measured from past population trends and used to generate tens of thousands of alternative future population trajectories. These simulated outcomes are then used to derive percentile projections, providing a quantitative representation of uncertainty around the population forecast.

The LGA forecast bands are derived with the same population concordance (or apportionment) approach used to produce the central LGA forecast.