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In a lonely hut on the edge of Ruapehu's crater, William Pike and James Christie heard the eruption as a "massive boom".
The door of the hut was blown from its hinges and mud and rock poured inside. One huge rock smashed into Mr Pike, crushing his leg.
Waikato DHB this afternoon said he was stable and awake - and had been communicating with his family.
As the 22-year-old lay in agony, he felt his life ebbing away. He asked his companion to tell his parents he loved them.
It was 21-year-old Mr Christie's first time on the mountain.
Before the eruption on Tuesday night, he, Mr Pike and two other companions had two days of "wicked weather, some great climbs and a lot of fun", he said yesterday.
The two men were in the Dome Shelter, 700m from the crater, when the eruption occurred.
Mr Christie said Mr Pike's right leg was trapped under a rock, and no amount of digging would free him.
"I put my hand down to move him; I couldn't move him. I thought I felt a piece of wood, but I'm pretty sure it was bone sticking out of his calf."
During the terrifying ordeal, the men talked, trying to reassure each other they were going to make it through the night.
Mr Christie said the conditions were freezing and everything in the hut was wet. He knew he needed to get help for Mr Pike.
"So we put his down jacket on and chucked some clothes around him like scarves - it was quite difficult because everything was wet and we weren't quite sure what to do.
"As I left, he said 'Tell my parents that I love them', and I said, 'You'll be able to tell them that yourself'."
Mr Christie said he ran as fast as he could down the mountain towards Whakapapa Village.
He ran into snow cat driver Shane Buckingham, who had just had a narrow escape from a lahar.
Mr Buckingham said he saw a "thick, dark, black thing" rushing towards him.
He realised it was a lahar, and that he was in grave danger.
He drove his vehicle at full throttle to higher ground, and the 8m-wide river of mud and debris missed him by about a metre.
He phoned his boss, Steve McGill, and Department of Conservation manager Harry Keys to tell them what happened and was told to "hang tight".
Mr Buckingham waited about 15 minutes.
"Then I saw this figure coming down the side of the mountain, zig-zagging toward me. I turned off my machine and heard him yelling 'help me, help me, my mate, my mate'.
"Something major was happening, and I got on the phone to my manager and told him this guy was running at me."
The man was traumatised and in shock, Mr Buckingham said.
Between panicked breaths, he was saying that his mate was trapped in the Dome Shelter.
"He said they were asleep in the hut, and they woke to the sound of rushing water and rocks coming through the door."
A rescue team of five was mobilised immediately, headed by Ruapehu Alpine Lifts ski patrol leader Andy Hoyle and including two other Alpine Lifts patrollers and two DoC staff members.
As Mr Buckingham drove Mr Christie down the mountain, he relayed information to the team.
Two snow-cat drivers drove the rescuers to the crater lake.
Although Mr Hoyle knew another eruption was possible, he was confident the mountain was safe after receiving advice from GNS Science and DoC staff.
Mr Pike was trapped by a boulder slightly larger than a tenpin bowling ball.
"It had come through the front door; the rest of the shelter was reasonably intact. We treated him for injuries to his legs and hypothermia."
Mr Pike was semi-conscious, but could not speak.
It took about 25 minutes to free him, get him warm, and prepare him up for the trip down the mountain.
Ruapehu Alpine Lifts general manager Dave Mazey said the snow-cat drivers involved in the rescue were two of the skifield's most experienced and more than capable of taking the machines to the top of the mountain.
"The terrain is not terrain we normally go into because it is outside the groomed slopes and the snow pack changes. But the biggest risk they were putting themselves into was from the volcano itself."
Last night Mr Pike, an Auckland primary school teacher, had regained consciousness and was talking to family and friends.
He had had his right leg amputated below the knee.
His father, Barry Pike, said his son was aware of what was going on around him.
"This is a miracle," said Mr Pike.
His son was still suffering the effects of hypothermia last night, and has damage to his left leg.
Dr John Bonning, the Waikato Hospital emergency department doctor who treated William at Taumarunui Hospital and then at Waikato, said he was amazed at his recovery.
"He has recovered from the worst degree of metabolic derangement that I have ever seen.
"His body had pretty much shut down and he was so close to death ... literally seconds from death."
Dr Bonning said William's body temperature was 25C to 26C and he was very close to cardiac arrest.
"For the majority of the night he was a hair breadth away from dying. It brought tears to my eyes to see him just now.
"He was so lucky it was just his legs. If there'd been any delays in any stage of the process, he would have been dead. Someone was with him every step of the way," said Dr Bonning.
"I congratulate everybody for what they did - to get him off the mountain, to Taumarunui, to Waikato. A young life with so much potential has not been wasted."
Mr Pike and the rescue team were met by an ambulance at the base of the skifield, in the area known as the Top o' the Bruce, and Mr Pike was transferred to the Iwikau medical centre.
He was further stabilised, and attempts were made to control his hypothermia.
"We set about trying to warm him with thermal blankets, blankets and hot water bottles," St John area manager Graeme Harvey said.
Mr Harvey said Mr Pike's hypothermia was severe, and his leg injuries were assessed as critical.
He was taken by ambulance to National Park township, where he was picked up by the Lion Foundation Rescue Helicopter.
The helicopter did not land at Whakapapa because of the risk of flying into an ash cloud.
"We didn't want our engine to stop," pilot Dan Harcourt said.
Mr Pike was flown to Taumarunui Hospital, where a specialist team did further work before he was flown to Waikato Hospital, where he had surgery yesterday.
Police area commander Inspector Steve Mastrovich, one of the co-ordinators of the rescue, said everyone involved had shown huge dedication.
"It certainly wasn't an exercise that wasn't without risk, but they just trudged up and did it."
Earlier, Mr Pike's mother Tracy described her son as a fighter and declared he would pull through.
Bebo site for Wiliam Pike
