Generally speaking, you won't be able to buy a new Garrett AT Gold from a US dealer due to restrictions on who they will sell to (ie: Foreign countries that have existing Garrett Dealerships). So you have to go a third party shipping company to purchase the detector on your behalf, and then forward it on to Australia (for a fee). If you do purchase from the US, the warranty will not be honoured here in Oz, only if purchased from an Australian dealer, otherwise it will need to be sent back to the US for any warranty claims.
The US AT Golds are the same as the Australian sold units, so there shouldn't be any fear of being ripped off - there aren't any AT Gold copies/counterfeits that I know of on the market.
As Dig'n-it mentioned, I've yet to see any gold found on this forum with the AT Gold. Doesn't mean that it is not capable, just a hell of a lot harder than with a pulse induction detector, and there will be limited areas on which it will operate without having issues with mineralisation, hot rocks, etc. Probably better off spending your cash on a more specialised high frequency VLF if you cannot afford the move to a pulse induction detector (Whites GMT, Makro Gold Racer, Fisher Gold Bug 2, Minelab Eureka).