With exploding visitor numbers and 125,000 people attempting the Tongariro Alpine Crossing annually, police are warning a death on Mt Tongariro is not a case of if, but when.

That follows the rescue of two trampers, one suffering hypothermia, in the early hours of Sunday morning in freezing conditions on Mt Tongariro.

Taupo Senior Constable Barry Shepherd said police at search and rescue headquarters were extremely worried the wet and cold woman was in imminent danger of dying.

She could no longer walk and was huddled in a sleeping bag on the side of a tramping track in the dark.

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He said the whole rescue could have easily been avoided if the group had prepared better, made better decisions on the mountain and stuck together. But there were several other factors which led these hikers so close to a meeting with death.

The Crossing has become an increasingly attractive draw after the Lord of Rings films set neighbouring Mt Ngaruhoe as the imposing Mt Doom.

The party of four in their late twenties and early thirties had set out from Mangetepopo late on Saturday morning, intending to tramp to Oturere Hut on the other side of the mountain.

It took them more than five hours just to reach Red Crater, a walk which normally takes less than half that time.

Despite the sun setting, the group carried on down to the Oturere Valley where they met blizzard conditions.

In the snow, the group became separated.

The first two men reached Oturere Hut, but the man and the woman left behind were wet and became cold. They called police at 8.40pm.

Using the 111 location system, police were able to pinpoint the couple's location and began a search and rescue operation but were unable to get a helicopter in because of low cloud.

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Instead, LandSAR volunteers Luke Middleton and Brett Donaldson were flown to Ketetahi Hut just before midnight to begin the three hour walk across the top of Mt Tongariro to Oturere, with another team 40 minutes behind them.

Meanwhile, police managed to contact the two other members of the group at the hut, told them of the couple's location and ordered them to go and find their friends, which they did.

It took the three men more than two hours to carry the immobile woman 1km back to the hut.

The searchers arrived at the hut a few minutes after the group and were able to use the spare warm gear they were carrying to get her changed into dry clothes and into the two remaining dry sleeping bags.

The rest of the group and the searchers spent a cold night in the hut, which did not have a working heater, and were helicoptered out at about 8am.

Mr Shepherd says the group made a series of mistakes which could have easily led to a fatality on the mountain.

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Firstly, they had set off too late and then failed to turn back when it became obvious progress was too slow to complete the walk in daylight.

They had taken crampons, but ones unsuitable for their boots and lost valuable time trying repeatedly to refit them.

They failed to travel at the pace of the slowest member of the group and become separated.

The group at the hut failed to raise the alarm when their companions didn't arrive, even though they had a personal locator beacon and could have found cellphone coverage near the hut.

"People are reluctant to ring 111 because they think it's for emergencies, but we say if you need us now, call 111 regardless of what it is."

Mr Shepherd said the group were lucky there were LandSAR volunteers and rescue helicopter crew who were willing to sacrifice their own sleep and time and put themselves at risk to try to reach them on the mountain.

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In all, four LandSAR searchers turned out in the middle of the night, along with two Greenlea rescue helicopter crew, four police staff and two LandSAR volunteers at Turangi Police Station.

"It required a really serious commitment to actually save [the woman's] life - that was potentially where it was going."

Mr Shepherd, a search and rescue veteran of 27 years, said given the numbers of people visiting Mt Tongariro and the lack of preparation of many of them, a fatality on the mountain was inevitable.

"It bothers a lot of people who go up there regularly just to see what people are doing," he said.

"One of these days someone will die and we're going to be having this discussion in front of a coroner."