In the Reps your vote must be fully preferential for it to count [number all boxes 1 to ..] but in the Senate, which has proven to be the obstacle to getting the past two elected gov't mandated policies converted into Legislation, it is now only necessary to number 1 to 12 I think?
I stand to be corrected?
So if there are 6 or 7 on the Senate party ticket you want to vote for then that only leaves 5 or 6 other candidates, to choose to number, out of the 100 odd on the ballot.
However if it is a hung parliament as with Gillard in 2010 every Reps vote is critical.