About 20 people were hurt in the crash near the Tūroa skifield.

Passengers say the brakes on a bus apparently failed as it hurtled down mountain road before it crashed, killing a girl.

The Ruapehu Alpine Lifts (RAL) bus, carrying 31 passengers, overturned at the 9-kilometre mark on Ōhakune Mountain Rd, which runs 17km up Mt Ruapehu to Tūroa, just after 2.30pm on Saturday.

Passengers said as the bus gathered speed, they prepared for the worst.

An 11-year-old girl died shortly after the bus flipped, throwing passengers on to the road.

About 20 others were injured, at least four with moderate or serious injuries.

Passenger Fenella Murphy told 1 News: "Every time the driver stepped on the brakes it just made a funny noise and we knew that something wasn't right.

FENELLA MURPHY "The whole experience was very traumatic," passenger Fenella Murphy said.

"There were people shouting, like, 'If you've got helmets, put them on'.

"Yeah we were sort of preparing for the worst and all the people who were sitting at the front, the children especially, were moved to the back of the bus."

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LIZ BROOKER A chopper flying away from the scene of the bus crash that injured several people on the way to Turoa skifield.

Murphy told Stuff: "The bus was travelling from Tūroa skifield heading back down to Ōhakune when the brakes suddenly failed and we were speeding down the hill for at least 30 seconds to a minute before we hit the side of the bank and [the bus] flipped and rolled, ending up on its left side on the side of the road.

"My girlfriend Aleisha Cope and I were the last passengers left at the scene to be taken by ambulance to the Ōhakune St John station, before being taken to Whanganui Hospital.

"Luckily we have walked away with minor injuries. However the whole experience was very traumatic," Murphy said.

"My heart goes out to the family of the child who didn't make it."

Aleisha Cope told 1 News it looked like the bus might go over a bridge. "I saw the bridge ahead and I thought, like, this is it. We're going to die ... we're going to go over that bridge and we're going to fall and this is it ... this is the end," Cope said.

Stuff has also been told by two other witnesses who arrived on the scene soon after the crash that survivors had said the brakes on the bus had failed.

LIZ BROOKER/TOICOMMS The crash closed the road and it wasn't expected to reopen on Sunday.

Those on the bus had allegedly been told to brace for impact moments before the crash, and one man had leapt from the vehicle just before it tipped over, two witnesses who helped survivors said.

When asked about the alleged brake failure, Ruapehu Alpine Lifts chief executive Ross Copland said the bus was taken to Whanganui to be inspected.

"The police and the Serious Crash Unit are investigating. I was not there, I don't know what happened," Copland said.

"I have not had a chance to speak to the driver ... We are all going through a process."

FIGHT TO SAVE GIRL

An 11-year-old girl died while being flown to Waikato Hospital on the Taranaki Community Rescue Helicopter soon after take-off from the scene. That chopper was then diverted to Taumarunui.

St John spokeswoman Chrissy Hamilton said two intensive care paramedics were on board the chopper working frantically to save the girl when she died.

"They would have been doing everything in their power to save her. Sadly, this is the reality of our work."

Hamilton said she and her colleagues were pleased with how the impromptu triage centre set up at Ōhakune had operated, and the other people injured in the crash had been treated as quickly and effectively as possible.

"The local community there really rallied around and helped us."

Waikato District Health Board spokeswoman Lydia Aydon said on Sunday morning that a 20-year-old man, a 29-year-old woman and a 31-year-old woman were taken to Waikato Hospital. They were all in a stable condition and recovering in the wards.

Ruapehu District mayor Don Cameron said the young girl's death was an immeasurable loss for her family. His thoughts were with them, and with everyone who experienced the ordeal.

"We are a small community, and we are in shock."

HELP CLOSE AT HAND

It was pure luck a St John senior manager happened to be on holiday in Ōhakune at the time of the crash, and was able to manage the setting up of a triage centre, St John district operations manager Steve Yanko said.

"There were multiple casualties who had to be dealt with at the scene. We were able to fly in paramedics and intensive paramedics and we got very good support from the ski people."

While no one was trapped in the bus, there were numerous people with fractures and possible internal injuries who could only be treated at the scene, Yanko said.

"Then there's the walking wounded, and those are the ones we got to the triage centre. There were quite a few people who were emotional and distraught and it was good to be able to take them to that calm environment."

After less than four hours, the triage centre had served its purpose.

"That was good, because the weather had started to work against us towards the end of the evening. I was really proud of my team. They did an excellent job."

There were about 1500 people skiing and snowboarding on Tūroa on Saturday, and it's believed about 1000 of them were stuck at the top of the mountain until just before 8pm.

Many of those crowded into the skifield's Tūroa Base Cafe.

Zac Williamson, who had spent the day up the mountain, said the top car park was jam-packed with at least 500 people waiting around.

BUMPER SEASON ADDS PRESSURE

Roam shuttle operator Terry Steven said an increase in visitor numbers this year had made an already tough task considerably harder.

"We're just under a lot of pressure with huge numbers, people are wanting to go up in any condition.

"The increase in business numbers is putting everyone under pressure, and pressure tends to find a weak point."

Drivers on both Whakapapa and Tūroa sides of the mountain had refused to take the public up the slopes on Sunday.

"We didn't operate today, out of respect to the family we thought it would be very disrespectful to be making money today, during such a sad time."

Ruapehu Scenic Shuttles owner Colin Baker said he had complained about safety issues on the mountain in recent weeks.

"The location of the accident just doesn't make sense … it's just not a place you'd expect to crash. I'm hoping for the driver's sake it was a mechanical failure."

*An incorrect file photo was initially used on this story. Stuff apologises for any distress caused by this.