With the major parties’ primary votes falling in recent years, preferences matter more than they used to - and they can be messy affairs.
Thanks to preferential voting, Australian House of Representatives members are each elected by an absolute majority of the voters in the electorate they represent.
With this election likely to produce a high number of non-major-party primary votes, the Greens have emerged as a strong third option and a headache for both Labor and the Coalition.
At federal elections, voters must cast a preference for all candidates in their lower house seat. Failure to do so, or failure to give an ordinal list of preferences, renders the ballot informal.
With the election result almost certain to be close, preferencing will play a key role, leaving the progressive parties in particular in a difficult bind.