Change Apple ID Region on iPhone or use 2 Different Apple IDs?

You will find that changing region is not so easy that you want to do so on a regular basis. For example, you have to cancel all your subscriptions. If you subscribe to Apple Music you will lose your playlists. Have a look at Change your Apple ID country or region - Apple Support

While Apple does not outright say your payment method must be from the same country as your country setting, my understanding is that in practice many have found this to be a requirement. It could be something more subtle such as the billing address country must match. It's highly unusual to have a card from one country billed to you in a different country.

It is possible to have media and purchases set to use a different ID, but the your device is still set to Australia, not the country of the services you are trying to use. I kind of think that would not work, in addition to:

Apple Media Services Terms and Conditions - Legal - Apple Media Services - Apple

- A. ”Our Services are available for your use in your country of residence (“Home Country”). By creating an account for use of the Services in a particular country or territory you are specifying it as your Home Country.

- I. “Certain Services and Content available to you in your Home Country may not be available to you when traveling outside of your Home Country."

There is no easy and good solution. One might suggest to the bank that they extend their app to also appear in the stores of other countries. Some have found the only solution is to have two devices, each set to a different country. Even then note that for streaming (which you do not mention), some times people find they cannot get streaming to work in another country.