The other four went for help and contacted police about 5.30pm.
Police, the rescue helicopter and St John paramedics - as well as 12 Ruapehu Alpine Rescue Organisation members - worked together in the dark to find the missing four. They were airlifted off the mountain about 8pm.
Mr Every said there was a "carpark" full of people on standby to help with the rescue, and a "full contingent" of rescuers prepared to climb up if bad weather meant the helicopter had to turn back.
"The four individuals were scattered down the western slopes of the mountain, unable to move from where their terrifying slides down the mountain stopped.
"By this time they were all separated and were perched on the icy slopes between 6500 feet and 7000 feet in altitude."
National Park Constable Conrad Smith said darkness had "well set in" by the time they found the group, and night vision goggles were needed.
Cloud also added to the hazards, but it cleared enough for the helicopter to get in.
"To give an idea of how solid the ice was, one of the rescuers used their ice axe to attempt to anchor themselves, however this twisted completely out of shape and was unusable," Mr Smith said.
"This was a very technical rescue with significant risks and the highly-skilled rescue team did a fantastic job given the conditions."
Mr Smith said it was unlikely the group would have made it to the morning given the cold and the wind chill which was "well below zero".
"Unfortunately the party of eight were not experienced, unprepared for the elements, and were using gear that was not suitable.
"I can't stress enough that being properly prepared could save your life. Winter is here and winter alpine conditions can be dangerous, and it's paramount that the correct gear and experience is required to go with that environment."
Mr Every described the rescue as a "stunning effort".
"It was a bloody good thing to be involved in."
He and another crew member picked up some of the trampers from hospital yesterday and said some of them could "barely walk upstairs" and were covered in cuts and bruises.
One had stitches across his face. The fourth was transferred to Rotorua Hospital.
They were "extremely grateful", "humble" and "lovely, lovely kids", and gave Mr Every and the other crew member hugs with tears in their eyes when saying goodbye.
While he said the trampers had made some bad decisions, Mr Every said it didn't matter "who's right and who's wrong" and described them as "kids doing a good, wholesome activity" and "living life to the fullest".