How your population forecast is built

This article is for Population Forecast customers. Using Placemaker? See: What is the National Forecasting Program?

Your population forecast comes from .id's National Forecasting Program, which combines a national demographic outlook with detailed evidence about housing development in your area. It is not an extrapolation of past growth. Each forecast is built from assumptions about births, deaths, migration, dwellings and households, constrained so that every area's growth adds up to a realistic regional, state and national total, and published as a single most likely outcome.

Key concepts

The cohort component model

Starting from a base population organised by sex and single year of age, the model ages all single-year cohorts year-by-year, applying deaths and migration to each. Births get added in every year, creating new single-year cohorts which have deaths and migration applied to them as they age in turn. Therefore, the forecast reflects both an area’s starting point, in who already lives there and how they will age, and how many and what kind of people are expected to arrive.

The housing unit model

We model the supply of homes available for occupation in each area: the existing stock, plus new construction, less demolitions, multiplied by an occupancy rate, which is the share of dwellings that are lived in rather than vacant. Applying an average household size to occupied dwellings gives the population those homes can hold. This is why housing assumptions matter so much to a local forecast: in most areas, people can only arrive if there are dwellings for them to occupy.

Top-down constraints

We set the national and state totals first, determined from demographic fundamentals and the economic outlook, then share them down to regions (SA4s, the regional building blocks of the Australian Statistical Geography Standard) and on to local areas. Every area's forecast must fit within its region's total, so growth cannot be counted twice and local optimism cannot outrun the state outlook.

Development assumptions: major sites and infill

Future housing supply enters the forecast two ways. We track sites expected to yield 10 or more additional dwellings individually in our Residential Development Layer, with their capacity, status and likely timing. A site enters the layer when it meets one of four conditions: it is under construction; it is going through or has recently received formal development approval; it sits within a strategic land supply precinct identified in planning instruments or growth strategies; or it is assessed as underutilised under current planning controls. We estimate smaller-scale growth, under 10 dwellings per site, through an infill methodology that considers zoning, lot configurations, existing approvals and historic rates of infill development. Each local area’s housing supply relies on a varying mix of infill sites and larger-scale development: for example, metropolitan suburbs with quarter acre blocks and smaller are likely to rely on a larger share of infill, while former cultivated blocks converting to residential housing will be dominated by builds of 10 and more dwellings per lot.

How it comes together for your area

A forecaster works through your region area by area, matching local housing capacity against the regional population outlook. Each area is assessed on its own evidence: future supply, historical dwelling completions and its demographic character, such as whether it is a family area where children are leaving home, a growth area attracting first home buyers or a retirement destination. There is no single formula applied everywhere. Mapped capacity is also not the same as forecast uptake: the Residential Development Layer identifies potential capacity, including some speculative and very long-term sites, and the forecaster applies judgement about how much of that capacity is used, and when, within the top-down constraints.

We maintain the development assumptions from satellite imagery, development application portals and state and local government planning documents, and we check them with councils: your feedback on sites, yields and timing feeds into each review.

A single most likely outcome

We publish one forecast for each area: the most likely outcome based on the best information available at the time it was prepared, updated on a regular cycle as conditions change. There is no high or low scenario to choose between, which means everyone planning from your forecast works from the same figures. For a specific site, the practical reading is that the forecast reflects a judgement about that site's realistic timing and uptake, not a promise that its full mapped capacity will be delivered.

Where to find this in Population Forecast

Your Population Forecast site presents the results for your local government area and its small areas, with details of the underlying assumptions and methodology included so you can see the basis of the figures. We produce forecasts at a small area level, by single year of age and by individual forecast year, so you can aggregate outputs to whatever age groups and planning periods your work needs.

Frequently Asked Questions

Q: Is the forecast just an extrapolation of past growth?

A: No. Past trends inform the assumptions, but the forecast is built from the components of change (births, deaths and migration), the housing capacity of each area and the demographic character of who lives there, all constrained to a realistic regional and state outlook.

Q: A major development is planned in our area. Will all of it show up in the forecast?

A: Not necessarily all of it, and not necessarily soon. Large sites are tracked in our Residential Development Layer, but the forecaster judges how much of a site's capacity is realistically taken up within the forecast period, and when, based on its status, development trends and the constraints on total regional growth.

Q: Where do the development assumptions come from?

A: From satellite imagery, development application portals and state and local government planning documents, maintained continually and checked with councils at each review. Feedback from your council on sites, yields and timing is part of how the assumptions are kept up to date.

Q: Why is there only one scenario?

A: We publish the single most likely outcome based on the best information available at the time, and we update it on a regular cycle. One set of figures means a council's service planners, infrastructure teams and strategists are all working from the same forecast rather than choosing between alternatives.