My practice is to fit the style of English to the setting and the viewpoint character. So if I'm writing something modern-to-futuristic with a vaguely American character I'll use American English. If I'm writing something historical or fantastical, with a European or at least Old-World character, then I'll use British English. I avoid idioms or strong accents so it's really just a matter of setting the word processor to one style of spelling or the other. Since your settings and characters are Australian it would be jarring if they didn't have certain Britishisms. I don't mean go full Crocodile Dundee but at least be authentic.
In any case, written English is still more or less the same language around the world. Spoken versions can vary in intelligibility but it's not like, say, Spanish, where you need different dubs for different countries.