Australia is big enough that NZ and Australia players should have no problem finding a local game. Americans getting the game will not make a difference as matches are joined based off of your connection to the host. I know 6 people personally in my small town alone who have bought the game and who are all having issues finding any matches at all. My guess is it's a matchmaking issue, that for some reason hasn't been addressed and probably won't be until the Americans discover it's not working and have it sorted out then. I've seen enough people complaining to warrant a response from Microsoft or 343 but haven't heard fock all at this point.
Fingers crossed the Americans matter more than the rest of the world does and 343 fixes the issue