>They see it as some great holy mission from god to go out into the hinterlands and convert the local people from their local existing belief system to their own particular form of Christianity

That is, pretty famously, the "great commission" given by the founder of the faith.

>Christian missionaries who treat the population of PNG as some sort of religious terra nullus

Apparently it's not a religious terra nullus, since PNG's parliament recently voted 80-4 to identify themselves as a Christian nation.

https://www.rnz.co.nz/news/pacific/544665/papua-new-guinea-d...