The following table was provided by Antony Green at ABC (Australian Broadcasting Co.) and has not been checked/verified by us. The Australian House (but not Senate) is elected by what in the USA is called "instant runoff voting" (IRV) but in Australia is called "the alternative vote."
|Number of Seats where|
|Election||#Seats||Average #Canddts per Seat||Combined Vote% for Minor Parties||IRV "runoff" needed (i.e. no majority in 1st round)||Plurality→IRV result "flip"|
Green remarks: As the above table shows, the number of seats requiring going beyond the first round of the instant runoff process increased substantially in the last two decades, mostly due to the rising vote% for minor parties (and the increasing number of candidates). The presence of party names on ballot papers since 1984 has also given voters more clues on who the minor parties represent. As anyone who has scrutineered will tell you, some voters give preferences to every other candidate on the ballot paper before finally making the choice between major party candidates.
Another remark: it is a theorem that, if at most 25% of IRV voters rank minor-party candidates top, then one of the top-2 parties will always win the election. Because the minor-party vote% has never exceeded 20.4% nationwide, it is not all all surprising to see massive 2-party domination in the Australian house. The last three elections in the table elected a total of zero minor-party seat-winners out of 450=150×3 total seats.
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