Yes, RCS in Messages is Apple adding support for a cellular carrier messaging account.
- If the carrier doesn't have RCS enabled for iPhone, you won't get RCS.
- If they have features not supported in their RCS implementation, you won't have those features.
- If they have limited the size of shared photos and videos, they will get sent shrunk/blurry.
Google was the one pushing RCS because they sell a server for it, and they integrated phone support for it. They also happened to have failed at well over a dozen prior chat attempts, and RCS was the one which stuck for them. But it wasn't until carriers said they wanted iPhones to support RCS that it went in - often externally influenced by the government - that iPhone support got implemented and started to get rolled out.
Sounds like in Australia, the big three aren't feeling pressure for universal RCS support.