GMW, I've done Australian characters before, and unless they're bush people, like Crocodile Dundee was portrayed to be, it's best to look at mainstream Australian television shows, news, and so on (widely available for free on the web) as one of the better ways to get the daily word usage and other idioms right. Surprisingly enough, which most people don't realise, is that the Australians (mainstream, not bush) sound surprisingly like an Americanised Brit. Well, to an American anyways. I was listening to a clip from a cop show from Australia, and aside from a few one in a billion "Australia only" words or phrases, and a slight, almost unnoticeable accent, they sounded just like I'd expect of someone from Washington DC or even Florida.
So it really depends on what you're shooting for as far as a cultural identification. For example, in the latest Madagascar movie, they take the European national identifications and lather them on thick, and I mean THICK. Okay, yes, it's a comedy, so going a little overboard isn't all that bad in a case like that. (heck, look at how weird they designed the characters in the movie. Freaky.) But the point is, what are you trying to portray from your character?
Another good example of choosing how far to stretch your character's culturally identifiable speech is to look at the Uncharted series. In the second instalment the two British characters were straight up, back alley, hard core London Brits all the way. If you've played the games, you'll notice that they fit perfectly with what the writer was after for them. Yes, they're a little cliche stereotyped that way, but it worked perfectly for the story.