WA Tomorrow 12 Population Forecast - Frequently Asked Questions

WA Tomorrow 12 is the official for Western Australian Government population forecast to 2036. The WA Tomorrow 12 population forecasts have been produced by the Department of Planning, Lands and Heritage, on behalf of the Western Australian Planning Commission.  

WA Tomorrow 12 is a series of trend-based population forecasts, by age and sex, for Western Australia and its sub-regions to 2036. The forecasts represent a best estimate of an area’s future population size and age-sex structure if trends in fertility, mortality and migration continue. They are produced using the latest estimates based on the five-yearly national Census of Population and Housing, and other demographic data.

The forecasts are used across government agencies to inform planning for future service and infrastructure requirements, including where a change in the age and/or sex distribution needs to be considered. The forecasts also inform diverse land use planning, public policy and analysis of development within the Department. 

These population forecasts cannot foresee future events that change current trends. These include significant shifts in government policy (such as migration), natural disasters and epidemics. Future planning decisions, such as infrastructure investments and land use plans, will also change population patterns including growth and distribution.

WA Tomorrow 12 has been released in two parts. The first release provides a whole-of-State forecast of the future population by age and sex to 2036.  It also includes a forecast range (Bands A to E), indicating five potential future population outcomes for the State.  

The second WA Tomorrow 12 release comprises the sub-State population forecasts by age and sex to 2036 for two geographic scales - Statistical Area Level 2 (SA2) and Local Government Area (LGA). The sum of each small area forecasts is equal to the WA Tomorrow 12 State forecast. 

WA Tomorrow 12 uses a Cohort Component Model (CCM) to produce the age by sex population forecasts for the State and its sub-regions. The CCM is the most widely accepted approach used by demographers for preparing population projections. The model is based on a series of assumptions that underpin what is likely to happen in the future with regards to the demographic components of births, deaths and migration. The assumptions are then applied to the baseline population, by age and sex, to produce the forecast population, age-sex structure, and estimated future number of births, deaths and migrants.

The cohort component model uses the following demographic equation: 

New population =

Base population + births – deaths + in-migration – out-migration

Further information can be found in Methodology.

WA Tomorrow 12 is a medium-term trend population forecast. Specifically, the future assumptions for the population forecasts are informed by a detailed analysis of recent trends in the demographic components of births, deaths and migration. Trend-based population forecasts are better suited over the short to medium term. There is a decreasing likelihood that these trends continue into the long term, due to diverse issues including new societal patterns, government policy and plans, economic cycles or natural disasters, which can change trends. 

The WA Tomorrow 12 sub-state forecasts have been produced for two geographic scales - the Statistical Area Level 2 (SA2s) and Local Government Areas (LGA). The sum of each respective small area forecast equals to the WA Tomorrow 12 State forecast.  

SA2 refers to the Statistical Area Level 2. This is a standard geographic area produced by the Australian Bureau of Statistics as part of its Australian Statistical Geography Standard (ASGS) classification of Australia into a hierarchy of statistical areas.  The SA2 typically encompasses one or more suburbs in urban areas, and sometimes one or more local government authorities in regional areas. The SA2 generally has a population of between 3,000 to 25,000 persons, with an average of around 10,000 persons. Further information on the ASGS can be found here.  

WA Tomorrow 12 uses a hybrid SA2 geography that reflects as closely as possible the current ASGS 2021. The SA2s that contain little or no population have been amalgamated with adjacent SA2s, leaving only 239 Sub-state forecast SA2 areas from the standard 265 WA ABS SA2s identified. This geography is referred to as the WAT12 SA2s.  Further information relating to the WAT12 SA2s can be found under the Geography tab in the SA2 forecast data Excel file, including the full list of SA2 amalgamations.

How are the LGA forecasts derived?  

Local Government Authority (LGA) populations were not directly forecast using a Cohort Component Model (CCM). LGA forecasts by age and sex were produced via a geographical concordance of population by age and sex, from the SA2 forecasts. 

WA Tomorrow 12 uses bands to provide a forecast range of potential future population outcomes. The State forecast uses probabilistic methods to produce its forecast range. The range is presented as five forecast bands ranging from low (Band A), medium-low (Band B), median (Band C), medium-high (Band D) and high (Band E). Band C is most often used by users as it represents the best estimate of the future population based on past trends.  

The WA Tomorrow 12 Sub-State forecast range differs to the State in that it uses an ‘Upper Band’ and ‘Lower Band’ to illustrate uncertainty for each WAT12 SA2 and LGA population forecast. These bands represent the estimated variance calculated for each small area, approximating the 80th percentile Absolute Percentage Error (APE). The upper and lower bands for each area represent an 80% certainty that the actual population will lie between these forecast limits. The Central Forecast represents the best estimate of the future population based on past trends.  The Sub-State upper and lower bands give the same 80% confidence range as the Band A and Band E from the State forecast. 

WA Tomorrow 12 is predominantly a trend-based forecast, being what the future population is likely to be if current trends in fertility, mortality and migration continue. The WA Tomorrow 12 forecasts are distinct from targets or scenarios contained in other government plans or strategies such as Perth and Peel @ 3.5 million, which are based on a target population size and are generally longer term in their outlook.

A target can be a number over and above a forecast and is often aspirational. Targets allow for critical appraisal of what planning and actions need to take place to achieve them, especially if the forecast is considerably lower.  

A scenario can be useful to evaluate a particular scenario, for example “What happens when you change one or more of the components?”. 

Other public and private entities also produce medium to long term population forecasts. The ABS and the Australian Government’s Centre for Population both produce WA population projections as a component of broader national forecasting exercises. Whereas these use nationally consistent methodologies, WA Tomorrow 12 is better able to incorporate unique local factors and trends.  

The WA Tomorrow 12 median forecast (Band C) is higher than the most recent projections published by the ABS (Population Projections, Australia, November 2023), Centre for Population’s 2023 Population Statement, and Geoscience Australia's SA2 projections. These nationally based forecasts anticipated that the WA share of national net overseas migration would be close to that experienced over recent years, which were impacted by the closed borders during the COVID-19 pandemic. However, the WA Tomorrow 12 forecast anticipates that the future WA share of national net overseas migration will be higher than the ABS and Centre for Population forecasts and will align more with the long-term, historic average.