A full internal investigation has been launched into a "debacle" on the Tongariro Crossing that police said could have led to the deaths of two students.

The students from Te Wananga o Aotearoa in Hamilton got separated from their outdoor education group in icy conditions on Mt Tongariro on Wednesday while their leader had no idea they were even missing.

Police say the situation was a "debacle" and if the hypothermic students had not been able to make a cellphone call to get help, they would have died.

The 13-strong group set out to undertake the 19.4km walk with one instructor.

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But conditions on the mountain deteriorated and the group became separated.

Three young men took a wrong turn at the Red Crater Summit and wound up heading along a ridge to Mt Tongariro's summit in 70km/h winds, with icy rain falling, snow underfoot and visibility at just 10m.

Meanwhile, the main group carried on and the leader did not realise until they got to the Ketetahi Shelter, that three of her group were missing.

On the ridge, one of the students turned back, leaving the other two still ascending the mountain.

Some time afterwards, the cold and wet pair realised they were lost and running out of strength, with hypothermia beginning to set in.

They called 111 for help and police contacted three professional guides, who are also Search and Rescue members, who were on the mountain.

"The other two gave up, sat down in the snow and rang police saying, 'we're cold, we don't want to do this anymore'," Senior Constable Barry Shepherd of Taupo police told Fairfax.

Shepherd earlier told NZME that if the students had not been able to make a cellphone call, "it's fair to say they would have perished".

"It was a debacle."

Te Wananga o Aotearoa has suspended all similar outdoor activities while it carries out a full investigation, launched yesterday.

Chief executive Dr Jim Mather said an external outdoor activities expert would review the investigation process and its findings to ensure it was robust and independent.

He said the wananga was providing full support for the staff member and tertiary health and fitness certificate students, aged between 16 and 28, who were involved in the ordeal.

The group returned to Hamilton on Wednesday.

"We are still establishing exactly what happened, but our immediate priority has been to ensure our students and staff have the support they need to recover from this ordeal," Mather said.

He reiterated praise and thanks to police, search and rescue, the Ruapehu Alpine team and the Greenlea Rescue Helicopter.

"We are extremely grateful to these people for helping return our students to safety and on behalf of our organisation I acknowledge everything they did," he said.