Cannot change App Store Country
Solution 1:
A valid US postal address (could be your current hotel) is needed, and then you need to select NONE at the credit card - because you just buy some US iTunes gift cards, and load them into your new US Apple ID account, from where you'll pay for the apps you download.
And you can still use this US Apple ID when you are back in Australia - for app updates and new purchase from the App Store.