An Australian professor and his two Papua New Guinean colleagues have been released from captivity, more than a week after they were kidnapped in PNG's highlands. 

It comes two days after another woman who had also been taken was set free.

Professor Bryce Barker, who is an Australian resident and New Zealand citizen, was held alongside fellow members of his research team Cathy Alex, Jemina Haro and PhD student Teppsy Beni.

The group was doing fieldwork in a remote part of PNG's highlands near Mount Bosavi, on the border of the Hela and Southern Highlands provinces, when they were taken by a criminal gang from Hela Province who demanded a ransom for their freedom.

Announcing the hostages' release on Facebook, PNG Prime Minister James Marape thanked police and defence forces, local leaders and the community for their assistance.

"We apologise to the families of those taken as hostages for ransom, it took us a while but the last three has been successfully returned through covert operations," he wrote.

The kidnappers had originally demanded a ransom of 3.5 million kina ($1.5 million), however it is unclear whether they received any money in exchange for the safe release of the hostages.

Mr Marape said in his Facebook post that the release had been negotiated "with no K3.5m paid".

It brings to an end days of tense and traumatic captivity, negotiations, and a complex security operation involving PNG police and defence personnel, in consultation with the Australian and New Zealand governments.

How a highlands research trip took a horror turn

On February 12, Professor Barker shared a picture of his arrival in PNG's capital on social media, captioning it simply "Port Moresby".

His friend Cathy Alex, a highly regarded local program coordinator, replied: "Welcome to PNG".

The two would soon be reuniting and heading into the country's highlands as part of an ongoing archaeological research program with the University of Southern Queensland (USQ).

Professor Barker had previously spent six weeks living and working in and around Fogoma'iu Village in 2019.

After delays caused by COVID-19, the team was excited to return.

Their project, focusing on early human migration across the Great Papuan Plateau, has been praised by Mr Marape as important work to "protect and promote" the region "as a rich biodiversity and archaeological site".

While the researchers were working, a gang of about 20 men were making their way towards the group, coming from the neighbouring Western Province.

Police said the criminals had been travelling up from Kamusi to their home in Komo, in Hela Province, when they saw the researchers "by chance".

Professor Barker and Ms Alex were taken hostage at gunpoint, along with two other members of their team: Jemina Haro, who works as an officer with PNG's National Museum, and Teppsy Beni, who has been completing a PhD through USQ.

Local guides who had been accompanying the researchers were also taken hostage. 

Hostages made harrowing calls from captivity

From captivity, Professor Barker used his satellite phone to call missionaries stationed nearby and pass on the kidnappers' demands.

The long-term residents had to act as intermediaries between the criminals and local authorities.

With no large sums of money in the remote area, the kidnappers were told the ransom would have to be sourced from Port Moresby and flown in by helicopter.

They were encouraged to ask for a smaller amount that could be sourced faster.

In a leaked recording of one call, Professor Barker told the missionaries that his kidnappers were threatening to kill Ms Alex if the money was not forthcoming.

In the call, it was said that one man had already been killed, but according to a leaked police report from last Sunday, villagers from the area reported that the man had only been wounded.

The kidnappers decided to release the local guides, but one of them opted to remain with the team. 

Villagers in the area who are not associated with the gang were able to monitor from afar and pass information to authorities about what was going on.

One local man told the ABC the gang, who had high-powered rifles, threatened to shoot the villagers when they tried to negotiate the release of the captives.

Police and army surrounded the area in days before

As the hostage crisis stretched on, authorities started pulling together a coordinated search and rescue mission. 

Extra security personnel were quietly being moved into the region.

Almost 50 heavily armed police were flown to Kamusi, from where the gang had set out.

The officers were to make their way up towards Fogoma'iu Village by foot, following the path often used by the criminals. A medical team from St John Ambulance was taken into the same area days later.

At the same time, defence personnel were marshalling in other areas.

Troops stationed in Komo were put on notice, and specialty personnel were flown in from Port Moresby to lead the mission if required.

Poor weather interrupted some early flight plans, and the days it took to bring extra troops and ammunition to the area caused concern among some involved in the operation, according to security sources. 

Some troops on the ground were keen to move in on the criminals quickly and rescue the hostages, particularly the three women.

But decision-makers had to maintain a delicate balance, prioritising the negotiation of a peaceful release to ensure the hostages got out safely.

Some additional army personnel from the capital were deployed to the regional city of Mount Hagen, before moving closer to the kidnapping site to wait in the nearby centre of Moro.

The defence troops sent in from Port Moresby took drones with them, ready to assist in the search for the hostages. 

Moro has the closest airstrip to the remote area where the researchers were working, which meant helicopters could launch from there to fly into the densely forested area where the hostages were being held.

The airport is managed by gas giant Santos, which operates in the region and housed representatives from the Australian and New Zealand high commissions as well as USQ staff during the final days of the operation.

Meanwhile, the gang was moving the hostages between locations in the area while demanding a ransom.

The police commissioner said the captives were being held in "difficult terrain", and security personnel raised concerns about Professor Barker's health.

One released as hostage crisis drags on

On Thursday, more than five days after the group was taken, word began spreading among locals on the ground that one hostage had either escaped or been released — Cathy Alex was making her way back to Fogoma'iu Village.

Police and the prime minister confirmed later that night that she was free and had been released by her captors.

The ABC has been told she was sent to pass on a message that the kidnappers wanted their ransom. After days in the remote jungle, it appeared the professor's satellite phone had died or stopped working.

In the wake of Ms Alex's release, PNG's police commissioner said authorities were "continuing to work to strengthen lines of communication, which remains a challenging aspect of this operation".

Ms Alex was said to have been weak from a lack of food and hours of walking. The following day, she was flown out of the village and then back to Port Moresby.

With the focus on freeing the three remaining hostages, other foreigners working in the region were being moved or contacted.

The ABC understands a British man who was also in the area was being protected by local villagers before being escorted out on Saturday.

Calls were also made to a French woman and two Belgians who were also in the region.

As the days stretched on and security personnel waited on negotiations, police worked to identify the members of the gang.

Pictures and names were compiled, and their families were contacted.

Announcing that 13 of the 20 suspected criminals had been identified, Prime Minister James Marape warned the gang there would be "no place to hide".

The gang had been terrorising the region long before the kidnapping, including staging attacks on local villages and attempted armed robberies at a logging camp in Kamusi.

How the group was saved

On Saturday, PNG's Police Commissioner David Manning flew to Moro as the negotiations were being finalised.

The ABC understands a ransom payment was discussed as part of the negotiations, although it was significantly smaller than the original amount demanded.

Authorities have yet to confirm whether money was paid or if any of the criminals involved have been apprehended.  

Professor Barker, Ms Haro and Ms Beni were flown to Moro, where a medical team was ready and waiting, before getting another plane to Port Moresby.

Professor Barker is expected to continue on to Australia.

Foreign Minister Penny Wong thanked the PNG government "for its leadership in securing a safe and peaceful resolution".

In a statement, USQ vice-chancellor Geraldine Mackenzie said the university was relieved to hear their much-loved colleague and his research team had been released.

"Professor Barker and his research team were in Papua New Guinea undertaking archaeological research," Ms Mackenzie said.

"Bryce is a highly regarded archaeologist and a valued colleague at the University of Southern Queensland and in the wider archaeological community. He has many years experience in undertaking research in PNG.

"Our deepest thanks go to the governments of Papua New Guinea, Australia and New Zealand, and the many people who worked tirelessly during this extremely difficult and sensitive time to secure their release."

But in PNG, concerns remain among those left behind in the area.

Villages there have long been terrorised by gangs from the neighbouring Hela Province, and many locals are worried the criminals may now be further emboldened or attempt to seek retribution on locals.