Genetic evidence indicates that for a successful founder population to be established, you've got to have a critical mass of around 400 people. You can do it with less, if you've got genetic mapping, but they didn't have that.....
Hawaii was populated. It seems to be more isolated, so the only thing differing is the stupidity of the protohumans. That would seem to be an asset in this case. There would be a dullness to the perception of danger and unless they paddled with one extremity, causing them to circle aimlessly, it would be inevitable that heading "south" would cause them to run into Australia. As to the lack of fossils, the standard answer is operant, they haven't been found yet. Still, I like the airborne dispersal theory the best and as the Hawaiian Rock Wallaby shows, you don't need no stinking 400 badges er.. specimens, three will suffice.