The first past the post system, ie the person who get the most primary votes wins, is not a very good system if you have more than 2 candidates. if you had 6 or 7 candidates a person with 20% of the votes cast could win.

When Australia set up their constitution (adopted in 1900 or 1901) it was lucky to have different models to work from (especially the USA and Great Britain). It is how the Aussie houses of Parliament are the House of Reps and the Senate, it was adopted from the US Constitution. A preferntial system of voting was also adopted because to win a seat you have to get more than 50% of the vote. This is done by voting for candidates on the voting slip in order of your preference. after counting, the candidate with the lowest amount of votes is excluded and his preferences allocated to the other candidates, then the second lowest till one person has reached more than 50% of the votes. Any close results are automatically recounted.

I think that Australia was well served by the gentlemen who drafted the constitution. Very few amendments have been made.