Not that strange, actually, once you understand the geography and ethnography. First of all (although not that significant), straight line distance between the population centres of AU and PNG is further than that of AU and NZ: Sydney to Port Moresby ~4 hours by direct flight, Sydney to Auckland ~3 hours. More significantly, the whole ~1000km length of Cape York Peninsula is one hell of a barrier: virtually uninhabited, no sealed roads most of the way, and extremely inhospitable (cyclones and mass flooding for several months every year, killer crocodiles / jellyfish everywhere, prevalence of dengue fever / malaria).

The Torres Strait Islanders are genetically and culturally part of the peoples of New Guinea, which is why here in Australia we refer to "Aboriginal and Torres Strait Islander peoples", why the TSIs have their own flag that's different to the Aboriginal flag, etc. We non-indigenous people of Australia and New Zealand share the same predominantly British ancestry, a very similar (and intertwined) colonial / post-colonial history, etc.