You bid in the currency of the auction. If you're on the US site and they're using something other than US dollars it will give the amount in the currency that the auction is in (in this case, Australian dollars) with the approximate equivalent in US dollars underneath. Sometimes sellers in other countries will list using US dollars, but I haven't seen that happen very often.
If the currency is something other than US dollars, you'll need to be sure that you can pay in that currency before placing a bid. If they offer paypal, this isn't a problem. And good luck!