Bear with me, this could take a while...
OK, so there's two elections going on simulateously - one for the House of Representatives and one for the Senate. Just as in the US, each electoral district (there are 150) elects a single member to the HoR, whereas each state elects multiple members to the Senate. In the case of the Senate though, each state is treated as one giant electoral district with proportional representation.
Both the HoR and Senate elections use single-transferable-vote ("instant runoff"), but this obviously plays out very differently in single-member electoral districts versus multi-member proportional districts.
HoR ballots tend to have a reasonably small number of candidates - between 3 and about 15, but usually somewhere nearer to 6. On these ballots, a full preferential ballot is the only option - you must number all candidates in order for your vote to count.
Senate ballots, however, are ballooning somewhat out-of-control. At the upcoming election, the Senate ballot paper in NSW will have 110 candidates. Requiring voters to number all of these squares for their vote to count was causing the informal ballot count to increase substantially, so in the 1980s "above the line" voting was introduced. Under above-the-line voting, you can simply indicate a vote for your preferred party ticket, and your full preferences will be allocated according to the ticket that the party submitted to the electoral commission prior to voting. These preference tickets are publically available, and the current controversy surrounds the party tickets lodged by Wikileaks in some states. "Below the line" voting is still available as an option (and obviously one I personally will be taking up).
There is growing recognition of the perverse results that "above the line" voting can cause, and of course 1-metre-wide ballot papers with over 100 candidates listed are absurd. There is some talk of allowing "optional preferential voting" above the line, so that you can just number the party tickets in your preferred order, rather than having to either number every single candidate individually, or accept your preferred party's direction of preferences in its entireity.