A young survivor of a canyoning tragedy walked in rain "literally like a waterfall" down to a gorge where six of his classmates would later die that day, says a coroner's report on the incident released today.

The report also says the group's instructor was not trained to take groups out of an escape route at a halfway point in the gorge, other instructors heard claps of thunder in the area and had seen "a brown raging torrent" coming out of the gorge, and that "under-estimation of risks" and "complacency" contributed to the students' deaths.

In April 2008 a group of students was swept down a flooding gorge and flung off a dam after their canyoning expedition became trapped by rapidly rising waters.

A coroner's inquest held over a week last month listened to the group's instructor describe the day of the tragedy and questioned her centre's chief executive about whether it could have been prevented.

"No appropriate assessment appears to have been undertaken as to whether there was a significant chance of the water level rising above a safe level during the trip. Such an assessment differs from 'keeping an eye out' for the water level rising during a gorge trip," said Hastings coroner Christopher Devonport in his 38-page report.

The group was from Auckland's Elim Christian College and their expedition into Mangatepopo Gorge in Tongariro National Park, near Turangi, had been run by the Sir Edmund Hillary Outdoor Pursuits Centre.

Ten students and their teacher were led by instructor Jodie Sullivan - in just her third month at the centre and sixth trip into the gorge - despite the rain.

One of the four surviving students, Kish Proctor, described the track he walked on to commence the gorge trip as wet, "literally like a waterfall" and said "it was heavy, heavy rain. It wasn't like normal, it wasn't easing up, it was heavy rain", Mr Devonport's report said.

On the same day, Robert Griffiths, another instructor at the centre, taking a group on a different expedition, radioed in that he heard "a loud clap of thunder".

Another of the centre's instructors, Rosalind Hughes, described the gorge later that afternoon as "a brown raging torrent".

Water had been up to her thighs just taking a group down a walking track. It wasn't normal, she said.

But Ms Sullivan had stated she could not determine what rainfall occurred while she was in the gorge, because of the overhang and narrow opening, and continuous dripping of water from above, Mr Devonport said.

The river's catchment had been parched and the falling rain rolled along its dry surface and quickly flooded the gorge - which went from calm with no danger signs to cloudy and roaring within an hour.

"When they commenced the trip the stream flow was low. By the time they attempted to exit the gorge at approximately 4pm the stream was a raging torrent," Mr Devonport said.

Sixteen-year-olds Natasha Bray, Portia McPhail, Tara Gregory, Tom Hsu, Anthony Mulder and Floyd Fernandes, and teacher Tony McClean, did not make it out alive.

The trip started at a dam and was a climb up the gorge to a halfway ledge and back - a distance of just 200m each way.

There was an escape route at the halfway ledge that required abseiling to get out but Ms Sullivan had not been trained for it, Mr Devonport said.

Ms Sullivan told the inquest in February how the group sang and prayed as cloudy water rose above their knees and they held on, unsteady on a ledge, with no immediate escape from the cliffs that closed in the gorge.

Eventually she decided to lead the young men and women down into the stream to try to get to the gorge's entrance, only 50m away.

A report of what happened written just days after the tragedy said Ms Sullivan had stood by a 7m dam at the entrance trying to catch the students as they came down the river one by one.

But all but two students were thrown off the dam on to rocks and massive logs. Two of them survived despite falling.

The bodies of the seven others were recovered downstream, two of them 3km away.

Kerry Charles Palmer, an outdoor pursuits field manager at the outdoor pursuits centre, told the inquest, held at Auckland District Court, he had discussed the rain with Ms Sullivan before she headed out.

"I asked her why she was still going into the gorge. She said she wouldn't go far," Mr Palmer said.

"I told her to check the river levels when she got into the gorge. But I wish I'd told her not to go into the gorge."

The centre's chief executive, Grant Davidson, apologised to the court unreservedly, saying a series of slippages in its systems led to the tragedy, for which he accepted ultimate responsibility.

Mr Devonport concluded: "Regrettably, lack of environmental awareness, lack of instructional use of historical information, instructor inexperience, lack of proper assessment before the gorge was entered to ensure there was no significant chance of water levels rising above a safe level during the trip, lack of or inadequate communication when in the gorge between the instructor and the Field Manager or OPC Tongariro base staff, failure to implement a crisis plan and dispatch response teams in a timely manner, under-estimation of risks, and complacency contributed to the tragic deaths of Antony McClean, Natasha Aimee Bray, Portia Caitlin McPhail, Huan (Tom) Hsu, Anthony Walter Mulder, Floyd Mariano Fernandes and Tara Rochelle Gregory in the Mangatepopo gorge on 15 April 2008."