I've just gone through and installed the latest Ubercart and played around with the options available in the tax module. Although very good and flexible I found your module actually comes closer to what we need. That is providing an end price for all users and showing the amount of GST included for Australian customers only, rather than adding it to the end price at the checkout.
However, from testing so far it looks like I need to customise it some more to make sure the GST is only shown if the customer selects 'Australia' in the billing details. Other than that... perfect. I will be uninstalling the default Tax API module and using yours.