I did not know that was an Aussie thing (heard in England often enough)

While I can understand this for muscular guys, I never got why it can also be used to mean a very curvaceous/attractive woman.

My favourite is “Going off like a frog in a sock”

But there are many to choose from. “Face like a dropped pie” is another gem.

As per usual, that’s British slang which was carried over to Australia. Almost all slang phrases Americans associate with Australia are British in origin and still in wide use, as you’d expect given the history of both countries.