I've been assuming the Australian marsupials have always been native to Australia. Wither Australia went, so went they.
The original Australian marsupial is believed to have gotten there after Australia was already a seperate continent. Australian marsupials form a clade which probably originated from one dispersal event from Antarctica.
I'm a bit confused about all this now, though. An article on
the origins of Australia's marsupials that I found while checking this contradicts what I said earlier about the marsupials out-competing placentals. It claims that
Tingamarra is the only identified placental from Palaeogene Australia, and that it's been disputed whether it's actually a placental. I may have been wrong about that bit.
Edited by caffeine, : Broken tags. And of course this post is pretty irrelevant, having been substantially ninja'd