I'm also in Australia, but I'm just another indie trying to muddle through my accounting obligations, so fair warning, I'm probably making mistakes about all sorts of stuff.
Everyone says get an accountant, and now that I can pay one, I have one! And he's great. He does the accounts for a bunch of Aussie indies and he knows all the stuff.
BUT when we first started out there is no way I could have afforded him. So instead I googled stuff and muddled through. All the info is out there.
First thing, you need an ABN in Australia. You can register as a sole-trader though, which keeps it simple. Its not necessarily the best way to structure a business, but it is the simplest. There's lots of info on the ATO website, and if you're confused about anything, call them - they're really helpful.
As far as paying tax in other countries goes (or even in Australia), I *think* a bunch of countries have tax treaties or something, which is why you only need to do an Australia tax return. Or maybe its just we're not making enough money for it to matter
Good luck to you! If you can afford an accountant, get one of them. They're great!