I am in Australia, bought a phone (Librem 5, received 2 years ago) and it works in Australia.
If you are specifically asking about carrier compatibility, see also: Cellular Providers · Wiki · Librem5 / Librem 5 Community Wiki · GitLab (scroll down to get to Australia).
I successfully tested the Librem 5 on the Telstra network and on the TPG/Vodafone network. I haven’t tested it on the Optus network. That is no way a suggestion that it won’t work with Optus, just that I haven’t tested it and hence I can’t make any claims.
One of the problems with reading old topics is that the information can be out of date. There was a time in the dim dark past when it was uncertain whether the Librem 5 would work in Australia (particularly in the bush) because at the time the “-T1” modem variant was not yet being offered. (That’s the variant you will need in Australia.) That consideration has long ceased to be an issue.
“My” topic covering (initially) any Australia-specific issues is at: [MyL5] Australia/New Zealand
It is my understanding that the modem cards for the Librem 5 USA are the same as those for the Librem 5. So there should be no differences between the two phones as far as mobile network issues go.
The core difference between the Librem 5 USA and the Librem 5 is that the former is assembled in the US, from components made in the US as far as is possible. The consequent difference is that the former is a lot more expensive (and that is an unpleasant difference with the way the exchange rate is at the moment). Only you can decide what your goals are in contemplating a purchase and hence which is best suited to you.