Just completed a trek in a pair of salomon xa pro 3d ultra (low cut) – I found them excellent. They did get wet inside and out but so did everyones. Keep in mind that these are designed as a trail running shoe so if you are looking at covering the trail in 6 days, they will allow you to maintain a solid pace. Whatever you end up going with, get in some long hard training in them on the highest, wettest most challenging hills you can find.

I don't think you can compare the Gold Coast Kokoda Challenge to the real one as you don't have the humidity, dozens of river crossings and kilometers of swamps to deal with, but if you've done GC in under 24 hours then 6 days shouldn't be a problem. Even quicker if you get a porter.