Which states and territories are likely to lose or gain seats in the 2023 House of Representatives determination?

The number of seats in the House of Representatives allocated to each state and territory is broadly related to each jurisdiction’s share of the national population (though it's not a precise relationship, as described below). To keep up with shifts in population distribution over time, the Electoral Commissioner is required to re-calculate the number of seats to which each state and territory is entitled a year and a day after the House first sits. The legislation governing this process is set out in the Commonwealth Electoral Act 1918. In the current parliament, the House of Representatives first sat on 26th July 2022; the next seat determination is therefore due on 27th July 2023.

The number of seats for each state or territory is calculated as the most up-to-date population estimate divided by the population quota. For the states, the latest Estimated Resident Populations (ERPs) are used; for the Northern Territory and ACT 'adjusted' populations are calculated which consist of their ERPs plus the populations of some small territories. The quota is defined as the aggregate ERP of the 6 states divided by twice the number of senators for the states (144). The result of dividing a state or territory population by the quota is then rounded up if the fractional part is more than 0.5, or down if it is 0.5 or less, and the rounded figure is the number of seats the state or territory is entitled to at the next general election.

However, there are some exceptions to this rule. The minimum number of seats for any state is 5, and for the NT and ACT it is 1 seat each. Tasmania is therefore allocated 5 seats (rather than the 3 it would receive otherwise). Both the Northern Territory and the ACT are subject to special rules if their unrounded seat entitlement is greater than 1.0 but less than 3.0. A recent amendment to the Electoral Act applies harmonic mean rounding. Effectively, this involves rounding up to 2 if the unrounded entitlement is between 1.3333 and 2, and rounding up to 3 if the initial value is between 2.40 and 3.

In July 2023, the latest ERPs available from the ABS will be those for 31st December 2022 in the National, State, and Territory Population publication. New population projections just published for the states and territories can be used to provide an indication of which jurisdictions are likely to lose or gain seats in this determination.

Based on the new population projections, the quota for 31st December 2022 is projected to be 175,677.3072. The populations and projected seat entitlements are set out in the table below. New South Wales and Victoria are likely to lose 1 seat each; Queensland may gain a seat, but it is too close to call because its projected population divided by the quota is very close to 30.5. Western Australia will probably gain a seat. The numbers of seats for South Australia, the ACT and the Northern Territory are likely to remain unchanged. The NT retains 2 seats only thanks to the new harmonic mean rounding rule. Tasmania will retain its 5 seats due to the 5 seat minimum provision.

A caveat should be stated here: these calculations are based on population projections, and actual population numbers available from the ABS in mid-2023 will turn out to be different to some extent. It is possible that COVID-related disruptions to migration trends may result in larger-than-expected revisions to migration estimates, and also non-trivial revisions to the preliminary rebased 2021 ERPs which were used as the starting populations of the projections. As a result, there is the possibility that actual seat entitlements will differ from those shown in the table.