That's more due to the proportional representation method (STV) used by Australia's upper legislative body. The lower body uses single-seat IRV and for most of

That's more due to the proportional representation method (STV) used by Australia's upper legislative body. The lower body uses single-seat IRV and for most of the past century has been strongly dominated by two parties. This isn't surprising because IRV (aka RCV) suffers from center squeeze, meaning that it tends to eliminate consensus candidates when there are three or more real contender candidates.