Just to clarify... there are a lot of items on the Amazon America site that are not available to people logged in with a non-American address. If you change your location (shipping address) to an American address (or postcode if you aren't logged in), the items will show up as being available. But it still doesn't make them purchasable to non-Americans.
I have a shipping forwarder as an alternative address and switching to it is how I can tell if something actually is in-stock, or how much it is to Americans (and gives me a backup for items not available to us if I really desperately want it, at any cost).
If you are logged in and have your shipping address set as being in Australia, some of those items that are not available to non-American addresses will have a box on the right side of the product page asking if you want to buy it through the Australian Amazon site, and it will send you to the Australian page when you click on it.