The answer to this can be found in Chapter 15. There, Diamond tells us that there were two main reasons why the New Guineans were protected to some degree.
First, New Guinea was home to diseases that Europeans could not deal with. Most importantly, there was malaria that prevented Europeans from settling in the lowlands of New Guinea for a long time.
Second, New Guinea is home to very rugged terrain. It was very hard for Europeans to reach the highlands in any large numbers until the 20th century.
Finally, European crops and animals do not thrive very well in New Guinea. European staple crops do not do well and even today, most crops grown in New Guinea are native crops.
For these reasons, it was harder for Europeans to conquer New Guinea than it was for them to conquer places like the Americas or Australia.