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
Most part of southern India, 'mutton' refers to goat meat. Sheep or Lamb have to be specified specially.
I grew up in Australia with mutton making up 90% of my meat intake, as we slaughtered our own merino wethers (so not the breeds of lamb raised for eating).
The local butchers would also have a decent stock of different cuts, particularly things like neck chops for stews. Numbers of sheep around are lower now, so that may have limited things a little.
I also think hogget is equally as or more common than lamb in city butchers. Some of those chops are huge and have a nice tang.
And on the Asia point, I'm pretty sure in India at least 'mutton' can refer to goat or 'lamb' of any age, typically goat, it's not making a rigid (regulated) distinction based on the age of the animal.
Lamb was reasonably common, but not in bulk like beef or chicken; was a fancy dinner option.