Abbreviated as ERP
Estimated Resident Population (ERP) is the official figure for how many people live in this country. The Australian Bureau of Statistics (ABS) puts it out. It covers all people who usually live here, no matter where they were born. The key idea is usual residence. This means anyone who has lived, or plans to live, here for 12 months or more. For people moving to or from overseas, the ABS uses a 12 out of 16 month rule to decide if they count as a usual resident. It includes people who are briefly abroad. It excludes short-term visitors.
The starting point is the Census, held every five years. But raw Census counts need fixing. First, a Post Enumeration Survey (PES) works out how many people were missed or counted twice. This is the net undercount. Second, people briefly abroad on Census night are added back. Third, the Census is in August but ERP uses 30 June, so a small fix is needed.
Between Census years, the ABS updates ERP every three months at the national and state level.
ERP(t+1) = ERP(t) + Births − Deaths + Net Overseas Migration (NOM) + Net Interstate Migration
Births and deaths come from state registers. Net overseas migration comes from travel records. Moves between states are tracked with Medicare data.
ERP figures come out in three tiers. Preliminary figures use early migration data. They come out after about six months. Revised figures use better birth and death data. Final figures use the full window needed to confirm overseas migration. These are the best figures between Census years.
After each Census, the ABS rebases the ERP. It goes back and fixes all the figures since the last Census to line them up with the new count. This gives us the best picture of how the population has changed.
ERP gives Place Forecast the official population figures shown to the left of the jump-off year marker on charts. The trend line comes from ERP data. When a user opens a population page, the trend line is built from ERP data.
ERP is also the starting point for all projections. The system takes the ERP by age and sex at the jump-off year as its base. Every projected birth, death, and migration event flows from this starting point. How precise the ERP is directly affects how sound every projected number will be.