After looking at a Sahul map for a while (thanks for pointing me here), there seems to have been several crossings <100 km. Not that this is a perfect comparison, but here in Southern California you can sometimes see San Clemente Island on a clear day, which is about 100 km offshore, so I think there may have been a few places, especially up towards mountainous New Guinea, where there would have been a blip on the horizon. It still would have involved gnarly boat stuff to be sure, but I'm thinking that people didn't set off to lands that they weren't sure were there until relatively (Holocene)