All my hosting is done in the US, but that doesn't mean any of my businesses are necessarily American.
If you specifically reject all customers attempting to sign up from an Aussie IP address, or with an Aussie physical address (if you have that), then you're on pretty firm ground to tell them to piss off if they come knocking.
But, y'know, I'm not a lawyer, and you might be subject to whatever whims any country cares to hit you with. Get some legal advice before trusting some random internet comment ;)
USA didn't like it though, and asked NZ to extradite him to face charges in the USA.
Legal battle still going, I think... but the business is dead.
I doubt Australia has that much clout, but you never know when an extradition will be the price of some favour to someone...