The deaths of six students and a teacher yesterday are the first fatalities in more than 30 years for the Sir Edmund Hillary Outdoor Pursuits Centre.
Thousands of students have enjoyed the buzz of canyoning at the outdoor education centre in the Tongariro National Park since it launched the activity as part of its outdoor education course in the early 1970s.
However, in 1976 a young girl died on the course after slipping under a log in the river.
Centre chief executive Grant Davidson said early this morning that he was "utterly devastated" by the tragedy and his thoughts were with the school and families.
Student from the Elim Christian College had been visiting the centre for a number of years, he said.
The trip the students were on was an "up and back" course, which involved them wading up the winding gorge from the bottom and then turning around and coming back again.
Canyoning is considered an adrenaline sport and involves scrambling, sliding or abseiling down riverbeds and waterfalls.
"It's a teamwork activity in an incredibly beautiful place," Dr Davidson told Radio New Zealand.
The students were wearing wetsuits, lifejackets and helmets.
The instructor which led the students yesterday was an experienced physical education teacher with a post-graduate certificate in outdoor education, Dr Davidson said. She had worked elsewhere before coming to the centre and undergoing training "some months" ago.
"We don't put people with groups who don't know what they're doing, and she was under the supervision of a very experienced field manager who had sanctioned the trip."
"It looks as if there has been sudden flash flooding which was difficult to foresee," Inspector Steve Mastrovich from Taumarunui police told Radio New Zealand.
"The instructor was with them and they were just caught at the wrong place at the wrong time."
Dr Davidson told TV3 News the water flow in the gorge rose from 0.5 cubic metres per second (cumecs) at 3pm to 18 cumecs at 3.30pm - "the equivalent to the Tongariro River going down a small stream" - before dropping back to 0.5 cumecs by 6pm.
"When they entered the gorge the water was at very low level and there was no prediction for heavy rain," he said.
"I am comfortable this was a normal activity we had with this age group in these sort of conditions. Obviously if we had known or predicted about the pulse of water we would not have been there."
The mood at the centre this morning was "very solemn", as families of the dead students started to arrive.
He told Radio New Zealand he had spent his life trying to avoid the situation he was now faced with, "but we are stepping up to it today".
All activities at the centre are now suspended and remaining students were being sent home by bus.
Dr Davidson said the centre would conduct its own inquiry and would co-operate fully with the police investigation.