The problem is that erectus would have had to be able to navigate at least 50 miles in the open ocean for them to get there, and I don't think our scientists are willing to accept that yet.
They were blown there as passengers on south sea palms, like Darwin's finches.
I have always read that myself. However, last week I saw a map of the region, during the ice age, that showed a connection between New Guennea(sp) and Australia. (The first one like that I'd ever seen, I'll see if I can locate it again)
On Java and Borneo you'll see human-like remains before that, simply because these islands were a part of the mainland during the ice ages. When Eurasia and North America were covered with ice, the ocean level was 150 meters (roughly) farther down.
Even with the water level that far down, land at the eastern end of the Indonesian Islands were still islands. That's the origin of the Wallace Line. Another very interesting subject.