It isn't as easy as an earlier post implies. Doing it that way won't let you download apps from both countries at the same time, it would just mean you would be using a different but single country for app and media downloads than the one used for everything else. It would still be only one country.
There is no real solution. Licensing is regional and Apple does not facilitate working around those restrictions. You can set up a different Apple Account, but if you have an Apple mobile device it can only be signed into one account at a time for the purpose of making purchases. Signing in and out of accounts can entail wait restrictions. Refer to this support article —> View and remove your devices that are associated with your Apple Account for purchases - Apple Support
‘When you try to associate a device that was previously associated, you might notice a message that says, “This device is associated with another Apple Account. You cannot associate this device with another Apple Account for [number] days.” You might need to wait up to 90 days to associate the device with a different Apple Account.’
The above statement is new in 2024. It used to be Apple only talked about restricting app downloads and streaming for 90 days.
It's not ideal but one way would be to buy a second Apple device and use that one with an account set to the other country. You don't even need cellular service set up on it if you are willing to restrict its usage to where you have wifi. Just use it for things like your Australian banking.