What a by-election is
A federal by-election is an election for a single House of Representatives division held between general elections because the seat has become vacant. Vacancies can arise for reasons such as resignation, death or disqualification of a member.
A by-election is different from a Senate casual vacancy. Senate vacancies are ordinarily filled through a separate constitutional and parliamentary process rather than by a public by-election for one Senate seat.
Who starts the process
The Speaker of the House issues the writ for a by-election, subject to the constitutional and electoral framework. The writ sets the formal election timetable, including nomination, polling and return dates. The AEC administers the election once the writ is issued.
Because the writ timetable is event-specific, current dates should always be taken from the AEC or Parliament rather than assumed from previous by-elections.
Voting system and candidates
The House preferential voting system applies in a federal by-election. Candidates nominate for the vacant division, electors in that division receive a House ballot, and preferences are distributed until one candidate has an absolute majority of the continuing formal vote.
There is no nationwide Senate ballot attached to an ordinary House by-election. This concentrates campaigning, media attention and political resources into one electorate.
Why by-elections attract national attention
A single seat can affect the government's majority, especially in a closely divided House. Even when the arithmetic of government is not at stake, parties often treat a by-election as a test of local sentiment, candidate quality or the popularity of national issues.
That political interpretation should be separated from the formal result. A by-election measures the choice of electors in one division under one set of circumstances; it is not a nationwide opinion poll.
How to verify the result
The AEC publishes candidates, polling places, counting progress and the final declaration of the result. Media projections can be useful on election night, but the official count remains the authoritative record.
For historical context, Parliament and the AEC publish material on past by-elections and vacancies. Those sources are preferable to unsupported claims about national swings or precedent.
Primary sources and further reading
We use authoritative Australian institutions as the starting point for this explainer. Follow these links for the underlying rules, official guidance and current details.