In my personal experience, mutton is not common in contemporary Australia. My local supermarkets (Woolworths and Coles) stock lamb, but I don’t remember either of them ever selling mutton. Maybe some specialty butchers might stock it, I don’t know, there isn’t much culture of eating it among the younger generations. That said, I imagine if I went back in a time machine to the 1950s or earlier, it would be much easier to find mutton in Australia.
Only time - that I can recall - ever eating “mutton” was in India - and I’ve been told that a lot of Indian “mutton” isn’t actually mutton, it is goat. You can buy goat meat here, most supermarkets don’t, but I’ve seen it in specialty butchers