To clarify, it doesn't really make sense putting the Liberals and Nationals, since they don't run candidates against each other (although they should, since there's no vote-splitting).
Actually in some electorates they do.
Where?
In some National Party strongholds (New South Wales and Queensland). I think they even do that in one or two electorates down here in Victoria. Though those electorates are close to the border (Vic & NSW) one.
Do they do it for nationwide elections? I know they do in some states.
There are always two or three seats both parties are contesting, almost always seats without a Coalition incumbent where an incumbent is retiring.
This is actually a big part of why the Nationals have declined to the point of disappearing federally; whenever a Nationals MP retires, both parties run candidates, and more often than not it's the Liberal candidate who makes it to TPP (even if the seat is lost, though as all but one or two Nationals seats are now ridiculously safe that doesn't happen).
Anyway, I have little passion for any Australian party now. Probably:
1. Democrats
2. Greens/Labor/Liberals depending on candidate
3. Greens/Labor/Liberals
4. Greens/Labor/Liberals
