As William Pike lay with his legs trapped under a pile of ash and rocks, he calculated he would die of hypothermia before rescuers arrived.

But the 23-year-old Aucklander defied the odds and a year later, spends little time reflecting on the eruption of Mt Ruapehu that cost him a limb.

"I just have an attitude of moving forward and really not dwelling on the past," he told the Weekend Herald.

On September 25 last year, Mr Pike and his friend James Christie were caught at the top of the mountain when it erupted without warning.

Advertisement

They were staying in the Dome Shelter when 2.5 million cu m of mud and rocks spewed out of the volcano.

The flow smashed into the hut and trapped Mr Pike's legs. Mr Christie went for help as soon as he realised he would not be able to free his friend.

After a daring rescue by a specialist alpine team, Mr Pike spent nine weeks in hospital and had his right leg amputated below the knee.

"I never look back on that day and think, 'Oh gee, I wish I wasn't there'," he said. "I was really enjoying myself, I was in my element and I was having a good time with James, and I wouldn't take that away for anything."

That positive attitude has taken the keen outdoorsman and primary school teacher far in the past year. He wrote a book about his experience and began what he hopes may be a side career in motivational speaking.

He has also launched himself vigorously back into sports, rock-climbing, kayaking and cycling, with the help of a collection of artificial legs.

He said he would be "forever grateful" to the Ruapehu Alpine Rescue Organisation team, the crews of the Taupo Lion Foundation Rescue Helicopter and Westpac Waikato Air Ambulance, and Waikato Hospital staff.

Mr Pike has clear memories of the eruption, and during the hour afterwards when he remained conscious, he calculated his chances of survival.

"The math that I worked out in my head wasn't looking good. I only gave myself about four hours to live, whereas I worked out it would take about six to eight hours for a rescue to arrive."

But despite his predicament he did not panic.

"I just thought if this is the way I'm going to die, I guess you've just got to go with it because there was nothing I could do. I was just grateful for the good time I'd had that day."

By the time the rescue team arrived at 12.30am, Mr Pike had severe hypothermia, and his body temperature was 25C.

"Waikato Hospital had not seen anybody with a body temperature as low as that apart from a dead person. I was extremely lucky."

Doctors told Mr Pike he would not have survived if he had not been so fit.