Well Denosovian DNA is basically absent on the Asian side of the Wallace line and at a very high frequency on the Australian side. On top of this there is physical evidence of homo sp. occupation of Australia as far back as 120K years ago which is before modern humans can be found outside of Africa.

There is one additional piece of evidence which is the modern Australian Aborignial and PNG population is descended from a founder population that appeared 15-30K years ago. Add all these up and the evidence is strongly in favour of the Denosovians being the first homo sp. that crossed the Wallace line.