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.
Two additional rescuers from Ruapehu Alpine Rescue Organisation were also called out.
Senior Constable Barry Shepherd of Taupo Police says the students' call was made at about 11.40am and the first rescuer, professional guide Terry Blumhardt, reached them about 3pm, with two other rescuers who had walked up from Mangatepopo Hut arriving shortly afterwards.
Blumhardt dug a trench in the ice and put up an emergency shelter and the trio force-fed the hypothermic young men with sugary food, electrolytes and hot drinks and wrapped them in foil blankets in an attempt to revive them.
Once they had warmed up a little and after two RARO rescuers had arrived, the group attempted to get the stricken students off the mountain.
The visibility made a helicopter rescue impossible and so the rescuers had to half walk, half drag the young men down to Central Crater to get them out of the wind, then walked them further down the mountain to below the cloud cover at Ketetahi, where they were eventually picked up by the Greenlea Rescue Helicopter about 6pm.
Shepherd said 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."
Former police senior constable Cliff Jones, an experienced search and rescuer who is now lead guide with Adventure Outdoors Tongariro, was one of the three rescuers who made their way to the stranded pair.
He criticised the group, saying they were "woefully under-prepared".
There was no pre-planning and although the bad weather was not forecast, the group should have turned back as soon as the weather deteriorated, he said.
One young man was wearing cotton long johns and although they had hats, gloves and jackets, the jackets were not waterproof, and they did not have waterproof leggings.
"For me, it's an example now of these under-resourced education institutions who take kids up into these areas and really don't know what they're doing and they could have lost those two boys so easily.
"They think the Crossing is just an easy walk, but it's not...these boys couldn't operate, they couldn't see, they were terrified and they were getting blown off the ridge."
Te Wananga o Aotearoa has since launched a full health and safety investigation into the matter.
Te Waenga Hoe Whakatere Jocelyn Mikaere said: "[The investigation] will include interviewing all staff and students involved. The recommendations from this investigation will be fully implemented."
Mikaere thanked those involved in getting the students back to safety.
"Te Wananga o Aotearoa sincerely thanks the police and search and rescue staff for their help in locating our students and returning them to safety.
"We acknowledge that without their help, this incident could have had a more serious outcome,'' she said.
Jones estimated the temperature on the ridge was well below zero. He said if help had not arrived, they would have died and it was touch and go whether they could even get the young men walking again and off the mountain before dark.
"This is one of the closest I have seen for a long time.
"These people haven't got the relevant experience or possibly the relevant training to be carrying out what they're doing.
"Somebody needs to be taken to account. They bloody near caused the death of two boys there yesterday."
Shepherd said the group erred by not sticking together, by not having a safety plan, and by not being adequately prepared.
Police had spoken to the leader afterwards and a representative of Ngati Hikairo, the Ngati Tuwharetoa hapu with mana whenua over Mt Tongariro, had also visited the Turangi marae where the group were staying to speak to them afterwards, he said.
Police would also be doing a follow-up.
With 120,000 people doing the Tongariro Alpine Crossing last summer, better awareness of the risks was needed, Shepherd said.
