I doubt any US airport vendor will accept foreign currency. I'm sure most major airports have a exchange company (if it's open) but you will lose value. Better to manage your AU dollars before arriving by spending them. We usually use local currency when checking out of the hotel (setting aside enough for transport to airport) and paying in cash first, then the balance w/no foreign fee credit card. Every hotel we've dealt with is very obliging to work it out that way.