Your device needs to be linked to Amazon US. You can do that in two steps:

1. In “Your Account” under “Address Book” add any US address (hotel, fictional).
2. Also in “Your Account” under under “Digital Management” go to “Manage Your Content and Devices” - from there you find “Country Settings” on the left, pick the US address

To shop Amazon.com.au change your ‘1-Click address’ to your Australian address. Go into 'Manage Your Content and Devices' and then into 'Settings'. Check that the ‘Country Settings’ say Australia. Click 'Change' to check your address is in Australia. Above your address, it will state: ‘You are currently linked to shop on Amazon.com.au.’ If it says ‘You are currently linked to shop on Amazon.com’, then click the related link to select Amazon.com.au.