Thanks for the tips! My goal isn't to visit all of these destinations in one trip, but my hope is to visit as many as possible as cheap as possible. I've changed my intial plan a bit, since travelling between islands seems kind of impossible in some cases. I'm thinking about spending about 3 weeks in December in Papua New Guinea and some of the islands around it. And then 7-8 weeks in January-February split in 2 with bases in Sydney and in New Zealand. From there I will travel to the islands and spend a lot of time in New Zealand as well.