Honestly, the best way to address this situation is to forego the duplicate-sites/-products approach and have just one site and one product range with Domestic and International shipping calculation configured appropriately. Just explain to us how you want your Domestic vs. International shipping fees to add up, we can tell you exactly what settings you would need to enter where -- here's some examples:
http://www.e-junkie.com/ej/help.shipping.htm
To address the problems you're seeing with your current setup:
There's no method to determine in advance that the country selected in the Cart will match the address they'll provide later during checkout, nor even to determine that the country selected in the Cart is accurate for their actual location. There are services that do a pretty good job of "guessing" where a person is located based on their IP address, but they're not perfectly accurate 100% of the time, so would you want to risk insulting a prospective buyer who's selecting their location honestly when the computer is telling them otherwise?
Okay, that said, how about just blocking completion of Checkout entirely, or sending the buyer back to the Cart for a correction, whenever there's a mismatch with the address they provide during Checkout? Unfortunately, not every payment processor provides a method to accomplish that; when they can support that, we support it too, and when we can tell the Checkout page to "lock in" the country selected in the Cart, we also do that.
Even so, what would the Aussie buyer do when they're sent back to a Cart that only lets them choose United States? They only wanted to purchase something from you and weren't given the option to select Australia, so they decided to forge ahead and see if they could request an Int'l shipment anyway. They probably don't realize you need them to visit a different site and rebuild their order from scratch there, so they'd likely just give up on ordering entirely.
I suspect the real problem is that people just plain don't "get" that you require ordering from one site for US buyers and another site for everyone else. If they're even aware of the two sites, they only see two sites that offer "free" shipping for the same products but one has them cheaper, so maybe they go for the bargain not realizing the difference, or maybe they only found the one site to begin with?
