The 50,000-year-old camp sites, in PNG's Ivane Valley, that are the subject of the Otago-led study were occupied during a relatively warm phase of the last Ice Age - the Pleistocene - when New Guinea was joined to Australia as part of the continent of Sahul.
Early humans were able to walk from PNG to what became modern Australia, until the sea level rose at the end of the Ice Age, separating PNG from Australia about 10,000 years ago.
The ancient camp sites, 2000m above sea level, are reportedly the oldest confirmed human settlement sites in PNG and Australia, and are also among the oldest known outside Africa.
Otago anthropologist Prof Glenn Summerhayes and colleagues have just published findings in the leading journal Science indicating that, as early as about 50,000 years ago, groups were regularly moving back and forth through extremely rugged territory to exploit rich plant food resources in the PNG valley.
Prof Summerhayes, the lead anthropologist, said that research resulting from the work of his team from New Zealand, Australia and Papua New Guinea could change the history of "modern" humans, showing that the people who once lived in the PNG valley were highly adaptable and at the forefront of global colonisation.
The research, supported by the Marsden Fund, was "rewriting textbooks" and overturning some old theories, such as the view that early modern humans spreading from Asia and across the Pacific were pre-adapted to coastal life and moved by simply "hopping" from coast to coast.
In this case, people had clearly ventured far inland and into mountainous terrain to gain high-value foods.
Northern hemisphere scientists had often focused on the spread of early humans from Africa to Europe, but the Otago-led research showed sophisticated modification of the environment in PNG by modern humans, 15,000 years before they colonised Europe.
"This is all new stuff - it's like going to Mars," Prof Summerhayes said.
"It's a bit like finding gold. Finding the earliest settlement [of Papua New Guinea] is something we didn't really go out to look for."
A Roman Catholic priest had first noticed "incredible" ancient stone tools being uncovered in the valley when foundation holes were being dug for a new mission building in the 1960s.
Otago-led scientists had gone to the area in 2005 and in several successive years, having to tramp in about 25km to the remote valley, which could not be reached by fixed-wing aircraft.
Further residue and micro-wear analysis of tools later found at the site by Otago-led researchers is being undertaken by Otago PhD student Anne Ford.
The team uncovered camp sites buried by volcanic ash where people made stone tools, hunted small animals and gathered nuts during the last Ice Age.
The findings suggested a deliberate modifying of the landscape, most likely to clear forest and promote plant growth.