Louisa Akavi's plight was a tightly held secret for five years, until the International Committee of the Red Cross (ICRC) on Monday broke the long silence after a desperate search following the fall of the last Isis stronghold found no trace of her.

The 62-year-old New Zealander was with the first group of Western hostages held by Isis, and one of a small group who remained in captivity after their fellow prisoners - James Foley, Steven Sotloff, David Haines and others - were brutally beheaded in a series of executions that shocked the world.

But while her captivity has long been known to the New Zealand Government, it has been kept a secret for fear any publicity would see her suffer the same fate as her fellow hostages.

The ICRC went public on Monday in an appeal for information to help find Akavi, who has an extraordinary record of service, returning to the frontline of conflict and war zones despite being caught up in violent situations, including a hospital massacre that slaughtered her colleagues.

It is believed Akavi was still being held by Isis as the end neared in the village of Baghuz, where a small band of the most fanatical Isis militants made their final stand. Stuff has confirmed that the New Zealand Government believed it had a positive identification of Akavi being alive as recently as December or January, as the remnants of Isis were holed up in a small town on the Euphrates.

ICRC has confirmed the positive sighting.

It said the search for Akavi, and two Syrians abducted at the same time - Alaa Rajab and Nabil Bakdounes - had consumed the ICRC for more than five years.

"At times, we've felt Louisa's freedom was close at hand," said director of operations Dominik Stillhart.

"At other times, the trail seemed lost."

Stillhart said Akavi had been held longer than anyone in the 156-year history of the ICRC.

"It is emotionally wrenching that a person so dedicated to helping others would have her freedom taken from her for so long." He confirmed that in the first years of Akavi's captivity, in late 2013 and 2014, the ICRC was in active communication with the Islamic State group.

"We were not able to persuade them to release her and that communication fell off."

It knew that Louisa was moved around a lot, including to Raqqa.

"A breakthrough for us came in late 2017, with the beginning of the end of [Islamic States'] broad control of territory, when people fled that region.

"We spoke with people in IDP camps in Iraq who had been treated by Louisa in Syria. From this we understood that she had been in places like Al Susah and Al-Bukamal (a.k.a. Abu Kamal) in late 2018, close to the Syrian-Iraqi border near the Euphrates River, the last concrete information we have on her whereabouts."

But when Isis was finally overrun there was no sign of her.

Akavi's remarkable story began October 2013, when she was abducted at gunpoint on a medical run for the Red Cross in Northern Syria.

She was passed from one militant group to the next before being held hostage alongside other Westerners used as bargaining chips in Isis' brutal propaganda war.

Highly-informed sources spoken to by Stuff over many years have confirmed it was likely Akavi's nursing skills that kept her alive, because of her usefulness to Isis as it came under heavy attack on the run from US and coalition forces.

Akavi is a veteran of the frontline and has served in countless other conflicts - including Iraq and Chechnya, where she survived a hospital massacre that left a fellow Kiwi nurse dead.

Akavi was taken hostage by armed gunmen who ambushed her medical convoy.

There were public appeals for her return by the ICRC, but while others taken hostage in the same incident were later released, she remained in captivity.

The ICRC says it does not know why the others were released, but not Akavi or the two Syrians.

In 2014, Akavi came agonisingly close to rescue - she was one of a number of Western hostages missed by only hours by US special forces in a raid to free them.

Over the following years she was moved multiple times. For a long period - with no sightings or fresh intelligence - New Zealand authorities believed she may have been killed.

But Stuff was told before the fall of Raqqa in 2017 that there was renewed confidence she was alive.

Like with other hostages, there were attempts to seek a ransom in the early stages of her captivity and proof of life was offered. But both the New Zealand Government and the ICRC have a firm policy against paying ransoms.

International media reports over the years have alluded to the Kiwi nurse, including her time sharing a cell with US hostage Kayla Mueller, a young aid worker, reported killed in 2015.

Mueller suffered brutal treatment at the hands of her keepers, including rape and torture.

One report suggested Mueller gave up her chance to escape her captors to look after the New Zealand nurse, who had shrapnel wounds and deteriorating health.

That report was based on interviews with a young Yazidi girl who shared a cell with Mueller.

Sources suggested to Stuff at various times that this is also what New Zealand authorities believe, though there are conflicting accounts.

Mueller's parents, Carl and Marsha, have told Stuff that other hostages relayed stories that suggested a special bond between the two while they were held together in Raqqa.

Kayla had at one point tried to protect the older nurse, who was sick and wounded. When the pair were split up, Mueller begged that she be moved with Akavi but their jailors refused and Akavi was kept behind to work in the hospital treating Isis wounded.

For years, successive governments have tried to locate Akavi and there were hopes of a rescue mission.

Stuff has been told about at least one aborted rescue attempt, though there may have been others.

Stuff has also learned through sources that at some point in Akavi's captivity, New Zealand military and intelligence personnel were placed on the ground with coalition forces in neighbouring Iraq to gather information and look for opportunities for a rescue.

At various times the "NZ Inc" team included soldiers from the elite SAS force, key GCSB and Security Intelligence Service personnel and police, in a Ministry of Foreign Affairs-led operation.

They remained there for the final battle for Baghuz and Stuff has confirmed some were relocated to a displaced persons camps in Syria, where it was believed Akavi might end up amid the chaos of a mass exodus from the heavy fighting and bombing.

They were on the lookout for Akavi and also any intelligence about her whereabouts. Their cover was lifted when Kiwi jihadi Mark Taylor revealed he had been interviewed by intelligence personnel after surrendering to Kurdish forces. Stuff has confirmed they wanted to know if he had come across the Kiwi nurse. He told them he had no knowledge of her.

The last known sighting of Akavi was December or January, when she was picked out of a line up of photographs shown to some of the tens of thousands who had fled before the final battle.

Stuff has been told that people were asked to pick her face out of a number of photographs of similar looking women, then asked to pick her out again, using a different pack of photographs.

The fact that people successfully did so meant there was a high degree of confidence that she was still alive and nursing Isis wounded as Baghuz came under heavy fire

But then there was silence.

In mid-April there was a surge of optimism when reports came through suggesting a woman matching Akavi's description had been spotted in a displaced persons' camp outside of Baghuz surrounded by Isis sympathisers. But those hopes were dashed when it turned out she was an Iraqi woman.

Over the following days as Baghuz and surrounding tunnels were cleared it became clear that Akavi was no longer there. Intelligence services officials briefed the Government on April 10 that Akavi's status was now "under review" - meaning they could no longer be sure she was alive.

Her family were asked to attend a meeting at the Beehive on April 11 and given that information. The family were also told it looked increasingly likely that the story could soon become public, and to prepare themselves for that moment.

Foreign Affairs Minister Winston Peters spoke to the family at that meeting and told them they should pray for her.

There has never been 100 per cent certainty about Akavi's survival. There were long periods without any news and some senior level sources spoken to by Stuff expressed doubt that some sightings were her.

The woman believed to be Akavi had worn a burqa and went by several Arabic names as well, so mistaken identity was a possibility.

In February, however, there were fresh reports of a New Zealand hostage after Britain's security minister, Ben Wallace, confirmed British journalist John Cantlie, also a hostage for more than five years, was still alive.

Other international media reports also referred to a New Zealand hostage - a nurse - who was also alive and being used as a bargaining chip by Isis in return for safe passage from the village of Baghuz, where a group of die hard fanatics were making their last stand. Those reports did not surface here because of agreements by New Zealand media not to put Akavi at risk.

Isis prisoners and family members spoken to after they fled the encircled town had confirmed three hostages - Cantlie, the New Zealander, and an Italian Priest, Paolo Dall'Oglio, 64 - were being held there, according to The Times.

None of them have been sighted since the fall of Baghuz, however, raising several possibilities.

The first - which ICRC appears to be pinning its hopes on - is that Akavi is alive, but caught up in the chaos among tens of thousands of Isis women, children and fighters at displaced persons camps.

Their hope is that publicity will focus attention on her and make finding her easier.

The second possibility is that Akavi and the other hostages have been spirited away with Isis leader Abu Bakr-Baghdadi, who has not been seen for some time. They would provide him with ongoing leverage.

The third option is that Akavi perished in the hell on earth that was Baghuz in its final days. But those who have followed her progress and extraordinary career, including 17 tours of duty, say if anyone could have survived, it would be her.

- Stuff