The Commonwealth Electoral Act 1918 provides three triggers for a redistribution of Commonwealth electoral boundaries. Under section 59 of the Act a redistribution shall occur:
- when there is a change in the representational entitlements of a state or territory
- when more than one third of the divisions in a state or territory vary from the average divisional enrolment for the state or territory by more than ten per cent for three consecutive months, or
- if seven years ('seven year rule') have elapsed since the last redistribution in the state or territory.
A redistribution of Commonwealth electoral boundaries occurred during 2002 and 2003 in Victoria, Queensland and South Australia. The redistribution in Victoria was occasioned by the 'seven year rule', while the redistributions in Queensland and South Australia were triggered by a change in the representational entitlements of the two jurisdictions.
The second trigger, malapportionment of electoral divisions, has not occasioned a redistribution since the three triggers were incorporated in the Act in 1984.
Worth reading for the impact of boundary changes, especially in Queensland and South Australia.
The Northern Territory missed a second seat by 295 voters. This was fixed by the House of Representatives (Northern Territory Representation) Act 2004 which restores the territory's second seat.