Where in Australia do most expats from South Africa live?

In my experience, Perth by far. Enough that the South African presence in Perth is very visible: it’s one of the largest ethnic subgroups there. (Perth has not had the same level of Mediterranean or Asian migration as Sydney or Melbourne.) South African restaurants are common there, and very uncommon in Melbourne (though I’ve still found a couple of places that had biltong.)

It helps that Perth is so much closer to South Africa, of course.