I don't know what the Australian version looks like behind the door panel. Might help if we had a pic with the door panel off.
On the US version, behind the door panel, there is a plastic housing that is attached to the metal door where the speaker goes. The plastic housing is sized to fit a 6.5" speaker, and the speaker goes into, and attaches to, that housing. However, even many 6.5" speakers require an adapter bracket to fit that opening or at least have to have their mounting holes re-drilled to fit the housing.
As I said, it would help to see a picture of what's behind your door panel. It could be that the 6.5" plastic housing is there, and just has an adapter bracket attached to allow mounting of a 4" speaker. If that's the case, you could just remove that bracket and you'd be starting at the same place as a US install. If that's not the case, you might need to fabricate something.
You can look at
www.crutchfield.com to see what they say will fit the US door. If you can take the door panel off and post a pic of what's there, we can compare it to the US housing and maybe get some ideas.
Agree with Black95Cam. If you just have a single speaker opening in the door, a 2-way would be the simplest install. However, a component set with separate woofer and tweeter would give you more flexibility with where to mount the tweeter for best sound. The component system would require additional wiring and a crossover that has to be mounted somewhere too, as opposed to just hooking a 2-way up to the factory wiring and being done.
As far as air leaks, you're going to have some air leaks in the door no matter what you do. And, you're not going to get kidney thumping bass out of the door speakers no matter what. If you want really good bass, you'll need to do something in addition to the door speakers, like a subwoofer in the trunk or 6X9s in the rear deck, etc. Again, just depends on how much money you want to spend, how good of sound you want, and how much work you want to do (or pay for).