How and when your population forecast is updated

A population forecast is not a one-off document. Your .id forecast is reviewed and updated on a regular cycle, so it keeps reflecting the latest population estimates, development activity and demographic trends. That means the figures on your site can and do change between releases, and understanding what drives those changes makes them easier to explain in reports and briefings.

Key concepts

Rebasing

Every forecast starts from a base population. The Australian Bureau of Statistics publishes Estimated Resident Population (ERP) once a year releasing population size for the previous year and adjusting population size for years prior. These remain estimates until once every five years data is released from the latest conducted Census, adjusting and finalising past population size figures. Rebasing means resetting the forecast's starting point to the latest estimates while maintaining previous growth rates: an upward/downward effect is triggered through every forecast year, depending on whether the starting population has proved to be larger/smaller than initially estimated. Therefore, even areas where nothing has changed locally will move when the base moves.

A forecast review

A review is a full, authored reassessment of the local areas in a forecast: the development site assumptions are checked and updated, and the population assumptions for the region, such as age structure and average household size, are reassessed. Council feedback on development sites is collected as part of each review.

Top-down and bottom-up changes

Because every local area must add up to its region, and every region to its state, a forecast can change for two kinds of reason: the wider regional or state outlook has shifted, so all areas within it adjust, or something local has changed, such as a development site being added, retimed or removed.

The update cycle

Forecast reviews run to a schedule rather than continuously, in two distinct steps. First, after each Census, forecasts are rebased: the base year is reset to the Census-updated population estimates and every forecast year adjusts with it, with the forecast horizon unchanged. Then a full authored review of every local area follows, which is when the development site and population assumptions are reassessed and the forecast range extends - for example, after the 2026 Census data is incorporated, the forecast horizon becomes 2026 to 2051. From there, local area reviews run on an annual cycle, so each council has a set window for development site feedback and its update. Your .id contact can confirm where your area sits in the cycle.

Why results change between releases

The common reasons are: 1) a new base: the forecast has adopted more recent ABS population estimates, shifting the starting point for every year; 2) a change in the wider outlook: the regional or state assumptions have been revised, and because all areas are constrained to those totals, local figures adjust even where nothing changed locally; 3) changed development assumptions: a site has been added, its yield revised or its timing moved, which changes the small areas it sits in; and 4) a shift off legacy product: forecasts prepared under our National Forecasting Program are constrained to regional, state and national totals therefore follow a top-down and bottom-up methodology approach, whereas forecasts pre-dating the introduction of NFP were prepared for a council area in isolation exclusively on the bottom-up approach.

Small movements can also appear in areas that were not directly reviewed. Average household size differs between areas, so when population is allocated across a region, the shares can shift very slightly even where the dwelling assumptions did not change.

One practical check before interpreting a change: confirm an update has actually been published. Forecast updates are timestamped on the site, so if your figures look unfamiliar but no new release is shown, contact us rather than assuming a revision has occurred.

Feedback and out-of-sequence updates

Council feedback on development sites is collected ahead of each area's review window, which is when it can shape the next release. If a forecast is missing something significant, an out-of-sequence update can sometimes be arranged, although the preference is to gather input at the scheduled review so each release reflects a complete reassessment.

Where this appears in Population Forecast

Your Population Forecast site shows the current published forecast for your area and its small areas, and details of the underlying assumptions and methodology are included with it so the basis of the figures can be checked. When a new release is published, this site will be updated and our key contact in the Local Government Council notified.

Frequently Asked Questions

Q: When will our forecast next be updated?

A: Reviews run area by area on a set schedule rather than on demand, so the timing depends on where your region sits in the cycle. The current release is timestamped on your site, and your .id contact can confirm your area's next review window and when to expect the request for development site feedback.

Q: Our population figures went down since the last release. Was the old forecast wrong?

A: No. Each forecast is the most likely outcome based on the information available when it was prepared. A lower figure in a new release usually means the inputs changed: updated population estimates are lower than before, revised regional trends showing reduced growth, or development sites that are progressing more slowly than previously assumed.

Q: Why did areas we gave no feedback on change slightly?

A: Areas cannot be forecast in isolation from one another. Average household size differs between areas, so when the region's population is allocated across them, small shifts can appear even in areas whose dwelling assumptions did not change.

Q: What happens to our forecast after the 2026 Census?

A: The forecast will first be rebased to the Census-updated population estimates, and full local area reviews will follow that to extend the forecast horizon to 2051. From there, reviews run annually, with each council having a set window for development site feedback.

Q: Can we get an update outside the normal cycle?

A: Sometimes, where a forecast is missing something significant. We try to avoid out-of-sequence updates because the scheduled review reassesses everything together, but if you believe something material has been missed, raise it with your .id contact.