I did some more research and flagged the issue to our Apple Support Community managers in the hope they can raise it up the line. It seems strange that Australian iPhone is listed as supporting 5G on all the same expected bands as everywhere else. I think the cellular service in Australia is compatible and there is a high probability it will be a copy-paste oversight on the web pages.
The best advice I can give is to wait until the real hardware hits retail stores in a week or two, then check again and if it is not resolved ask questions direct to Apple employees in Australia.
Unfortunately we don’t know yet whether you will get any kind of indicator on the watch display to say which cellular service it is using.