An Australian professor and his three Papua New Guinean colleagues who are being held hostage in the PNG highlands were "sighted by chance" by the armed group who took them. 

PNG Police say the criminals are "opportunists" who are demanding a ransom for the release of the researchers. 

The academic team was doing field work in a remote part of PNG's highlands near Mount Bosavi, on the borders of the Hela and Southern Highlands province, when they were taken.

The researchers were working on an ongoing project and were accompanied by local guides from the area. 

The guides were also taken but later released.

However, the ABC has been told that one of the local guides elected to stay with the researchers, despite being offered his freedom.

The country's Police Commissioner David Manning said "specialised security force personnel will use whatever means necessary" including lethal force to secure the release of the hostages. 

He is urging the criminals to release them, saying a "failure to comply and resisting arrest could cost [them] their lives".

Police believe the group that kidnapped the four are from Komo in Hela Province and were returning from the village of Kamusi when they came across the researchers.

"These are opportunists that have obviously not thought this situation through before they acted, and have been asking for cash to be paid," Mr Manning said.

The anthropology professor works for an Australian university and is a permanent resident, but is a citizen of New Zealand.

He has an archaeology focus and has done research work in PNG previously.

He is being held alongside three Papua New Guinean women: a highly regarded program coordinator and two University of PNG graduates, one of whom is believed to be a museum officer.

There have been concerns that other foreigners were also being held, including another Australian researcher, but it has been confirmed that person is home in Australia.

Hostages moved between locations

Police on the ground have been working with local missionaries who have been able to act as intermediaries to try to free the hostages.

The missionaries have spoken with the professor via a satellite phone and have been trying to help with negotiations.

The armed kidnappers had moved the hostages between villages in the area, according to the Assistant Police Commissioner for Western End John Kale. 

He said the operation is delicate.

"Police are planning to respond to the situation, but because the missionaries there are in communication with the hostages, or the people who took them captive, we don't want to create a situation that will become hostile," he told the ABC. 

He also raised concerns that the hostages could be moved again or harmed.

A businessman from the region said word of the kidnapping had quickly spread among locals who he said are keen to help, despite logistical difficulties.

"We are living in scattered villages in a very thick forest so we could not be able to locate them and there is no network there to contact them and identify them," said Andrew Awabi from the Mount Bosavi Sustainable Agriculture Development Initiative.

PNG Prime Minister James Marape said villagers nearby who are not associated with the criminal group have been assisting with negotiations and passing on information.

It is rare to see kidnapping for ransom in Papua New Guinea and Mr Marape said it represented "a trend of criminal activities we will not tolerate in our country".

"I think this is the first time a ransom is attached to a hostage in a situation like this and we're treating it very seriously," he said.

"We don't want it to be a precedent for the future, but we're working with the authorities concerned."

Second Australia held captive last year

There have been kidnappings in PNG in the past, but they often have a connection to resource projects, or industries working in regional areas.

At the end of last year, an Australian and three Papua New Guineans who were contractors for Santos at a remote oil field in Hela Province were kidnapped by some landowners, according to local media reporting of court proceedings.

That group was reportedly held hostage for several days after being taken by the assailants who were armed with knives and homemade guns.

The hostages were later released and their ordeal was not publicly known at the time. 

Three men are currently facing court over that kidnapping and told the court they took the hostages because they haven't been paid royalties by the PNG government for the oil project.

"We worked closely with PNG and Australian authorities as well as the contractor to reach a positive resolution," Santos said in a statement to the ABC.