Investigations into what led a bus to overturn and crash in Tongariro National Park, killing a girl, are ongoing.

The Ruapehu Alpine Lifts (RAL) bus, carrying 31 passengers, overturned at the 9km mark on Ohakune Mountain Rd, in Tongariro National Park, just after 2.30pm on Saturday .

The smashed bus is in Whanganui being inspected, while the driver of the bus has since been released from hospital.

The bus was carrying passengers down from the Tūroa ski area when the incident happened.

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The chief executive of RAL, Ross Copland, said he had spoken to the driver.

"He's really shaken up," Copland said. "It was a pretty traumatic experience for him."

"It's been widely reported from the passengers on the bus that something's happened there [with the brakes]. From our point of view that's what we're really interested in getting to the bottom of.

"We're running our own investigation alongside the police."

An 11-year-old girl died in the crash and Copland said the company had been speaking with the victim's family.

"I mean, I've got young children myself," Copland said. "It's beyond tragic. There aren't any words for it."

RAL was speaking to all of the passengers individually about the crash but it wasn't always easy to make contact.

"We're ... slowly getting in contact with the passengers and working through that with them. I've met with one of the injured passengers this morning.

"Obviously with the scene on site ... people lost cellphones and things like that so it's taking us a little while to make contact with everybody but we're progressively getting through that this week.

All of RAL's buses except one other Mitsubishi Fuso, taken off the road as a precaution, were still in operation.

"Yesterday was a pretty big day of review and debrief," he said.

"The important thing for us is to establish that the buses that we do operate is safe to continue and as part of that process we took the other Mitsubishi Fuso bus that we operate off the road."

There had been some speculation the Certificate of Fitness for the crashed bus expired on the day of the crash.

The New Zealand Transport Agency's database showed Saturday, July 28, as the expiry date for the certificate - the commercial and heavy vehicle equivalent of a warrant of fitness.

However, an NZTA spokesperson confirmed the bus had a valid Certificate of Fitness.

When police entered crash details of a vehicle into the database, the certificate expiry was changed to the date of the crash.

Copland hoped the company would be back to business as usual soon.

A passenger who was in the crash has recounted the chilling moments in the lead up to the incident.

The passenger, travelling with his brother, told the New Zealand Herald they had only been driving for around five minutes when the bus brakes failed.

The passengers started to panic as the bus picked up speed and began to sway as the driver attempted to steer it down the road.

"We just kept picking up speed, leaning further to each side ... it actually felt like the movie Speed," the passenger, who wished to remain anonymous, said.

One passenger got out of their seat, opened up the back doors and leapt out of the bus, while another told others to put their helmets on.

The speed kept increasing and the bus rolled on a corner.

At least 18 people were injured, including four with moderate or serious injuries.

The injured passengers were taken to Rotorua, Taupō, Waikato, Whanganui, Palmerston North and Wellington hospitals following the crash.