[wizofaus]

Neither, it's essentially a type of ranked voting that is often referred to as "two party preferred" on the assumption that for any seat, it will go to one party or the other, and it's likely to be one with the highest first+second preference count (not exactly how it works, but you can find the details online easily enough. A fairly typical scenario is that the ALP might have something like 40% of the 1st preference votes and the Liberals 45%, but the 2nd preference votes overwhelmingly favour the ALP and they take the win). There hasn't been any serious discussion over the need to revisit mandatory voting (personally I don't think it should be over a certain age - there's an argument those over 75, who are making up an increasingly larger percentage of the population, have undue influence on selection of a government whose job should be to put forward policies etc. that determine the long-term future of our country, which is of far more relevance to those who'll be alive to see it. I imagine once I'm past 75 I wouldn't be so concerned with voting in every election on that basis. But I'll admit it's not something I feel particularly strongly about.)

[ekimekim]

It's simply ranked choice voting (aka IRV) for the house. "Two-party preferred" is a system used by polls and news to try to convey how they think the runoff voting will play out, by picking the two candidates most likely to come first and second and showing their expected final tally.