The rescue helicopter at Mt Ruapehu after a person fell into the crater lake.

Mt Ruapehu's crater lake, which claimed a life over the weekend, would have been warm to the touch, but its treacherous surrounds are likely what killed, experts say.

The man died on Saturday, and the Department of Conservation (DoC) announced a rāhui - essentially making the upper part of the North Island's highest mountain off limits. The rāhui would stay in place until sunrise on Wednesday.

GNS volcanologist Agnes Mazot said the lake's temperature was between 32 and 33degC, a warm bath's temperature, over the weekend but the water was notably acidic with other nasty elements in it.

GEONET There is plenty of steep terrain around the Mt Ruapehu crater lake.

"It is not pleasant if you have a cut on your hand."

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But the real danger was the steep terrain around the crater, which included cliffs and a glacier, she said.

JOSEPH PEARSON/STUFF The man was recovered from the lake by members of the group but later died.

Mountain guide Gary Dickson said the crater was easy to access, helped in part by the skifield lifts which carried people a good distance of the way up. But the terrain was genuine mountaineering, even if it was at the easier end of the scale.

At this time of the year the ice and snow surrounding the crater could be turned into small, icy cliffs by the water and that would make getting out harder, especially without proper equipment, he said.

DoC said the rāhui meant people should not go above the ski areas' boundaries out of respect for the man who died.

SUPPLIED The Mt Ruapehu crater lake, where a man died on Saturday.

"This provides time for healing and recovery of the natural elements at place and people, in particular the grieving whānau," a DoC statement said.

Hazel Phillips, who was at Whakapapa as part of a group of ski tourers on Saturday afternoon, witnessed some of the retrieval.

"This is a very sad outcome," Phillips said.

"Conditions were windy and icy underfoot and this presents a sliding risk."

"We also witnessed someone fall while descending Dome, lose his ice axe and thankfully come to a stop on a flat part. He was badly equipped and not experienced."

He was lucky to not land in the crater lake too, she said.

"With spring conditions softening the snow above the lifts on Ruapehu, this is a high risk time as people who aren't used to serious alpine conditions begin to venture into unpatrolled and uncontrolled terrain.

"The Ruapehu Alpine Rescue Organisation and the RAL [Ruapehu Alpine Lifts] ski patrollers do an incredible job of helping people inside and outside of the back country on our maunga."

It was a timely reminder for all back-country snow users to ensure their skills were honed and their gear was correct, she said.

Emergency services were flown to the scene via helicopter at around 1.50pm on Saturday.

MetService's lead meteorologist Mark Todd said conditions were largely fine and sunny during Saturday.

In September last year, adventurer Richard Ebbett went missing after heading to the Whakapapa ski field, and his body was later located in the Crater Lake.

The death will be referred to the coroner.