Todd Donovan of Western Washington University writes to me about alternative voting or IRV and how this can be impacted by large numbers of ballots being counted after Election Day:
The long count of votes in Australia is somewhat complicated by the Alternate Vote (AV) system used there. Under AV (known as Instant Runoff Voting in the US), voters rank their preferences for all House of Representative candidates on their ballot. In the initial round of vote counts on election night only “ordinary votes” (cast at the polling place) are processed. The candidate with the least first preference votes is eliminated. A second round of counting then proceeds, with the eliminated candidate’s ballots being examined so that her supporters’ second preference votes may be transferred to the remaining candidates. These transfers continue until someone emerges with a majority.
But as pre-poll, postal, absentee, and provisional ballots are received after election day, the process is ran again. Since the final count can be contingent on the order that lower-ranking candidates are eliminated, everything is ran again from scratch – even in districts where a candidate had reached a majority in previous counts. In the vast majority of districts, the leading candidate on election night will win the seat. In a seat where the margin is razor thin, additional ballots and new preference transfers can cause things to bounce around a bit.