Not always, it does pay to check the AA AU site. It has substantially less inventory than the US site, but i've found a lot of fares to be the same or a few dollars less.
You can't use your AU address to pay for flights using a credit card on aa.com though. There are ways, but just do a search around here to find out how (or you can put your bookings on hold, call the AA call centre in BNE (at 0900 else you'll wait forever) and mention you have a booking held that you could not pay for, and the flights were not available on the AU website, and usually they'll waive the phone booking fee). You can also book it thru the UK site and pay in GBP.
AA are one of the better airlines at keeping fares reasonably similar when converting currencies. I've booked on all three (AU/US/UK) sites and never had a problem with any of them