Er... Steve Irwin certainly had a really strong, broad accent, the type that we cringe at when non-Aussies try to imitate it. I don't know about Bindi, but she's Australian through and through, so it's certainly not fake or exaggerated at all. It's been ages since I've heard her voice, but I think she pretty much sounds like your average little Australian girl.
And to your second question - nope. We're not at all like the US and UK in that respect. There are some slight dialect differences between states, but they're mostly just minor vocabulary issues. Pronunciation of a few sounds may vary slightly, but generally, I don't think you can tell where someone's from from their accent. You'd have to listen very hard and be used to Aussie accents to be able to pick up the few tiny clues there could be.