One passenger had a wild ride after a bus slid down a hill into an embankment in Hamilton.

A driver was seen running wildly after his runaway bus as it careered down a hill with one terrified passenger inside.

The Waikato Regional Council Orbiter bus rolled 200m down past the entrance to Hamilton's hospital before it plunged over a steep embankment.

It narrowly avoided a Waikato Hospital building with 15 staff inside.

DOMINICO ZAPATA/FAIRFAX NZ It's unclear whether the handbrake failed or wasn't put on properly.

Witness Matt Sayers, who was on the Waikato Air Ambulance helipad, saw the drama play out.

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"The driver had gotten out and 30 seconds later, it started rolling," Sayers said, "then we saw the bus driver running after the bus.

Dominico Zapata Fairfax Media NZ A bus crashed down a bank next to Waikato Hospital. It left a trail of destruction as it careered down the hill without the driver on board.

"He was never going to catch it. But he was running and running."

The driver had parked the bus at the hospital entrance and got out for a toilet stop.

A passenger was left alone in the vehicle at the top of the hill on Pembroke Street, which is used as a thoroughfare for ambulances getting to the emergency department, Waikato Regional Council operations manager Andrew Wilson said.

DOMINICO ZAPATA/FAIRFAX NZ Maiah Marchioni, 5, sat transfixed watching the crashed bus while at her grandmother's work for the holidays.

The passenger was uninjured but shaken, Wilson said.

It's uncertain whether the handbrake failed or had not been pulled up properly, he said.

On its 200m journey, the bus slammed into a retaining wall before veering over the road and plunging over an embankment.

DOMINICO ZAPATA/FAIRFAX NZ The ground bore the brunt of the impact.

Poles from the embankment's retaining wall were flung across the road.



The whole thing seemed to happen in slow motion, Lollipops Educare Waikato Hospital teacher Tanya Bavastro said.



She was sitting in the centre's staff room about 8.30am when she heard a big crash.



"All I heard was a big crash. Next minute, a bus came flying through. It slid into the building and crashed.

"It was in slow motion but really fast."

Bavastro dialled 111 while a colleague ran outside to check everyone was fine.

DOMINICO ZAPATA/FAIRFAX NZ The bus was parked at the top of the hill when it started rolling.

She couldn't see a driver, but any passengers on the bus "must have been terrified".



Seeing the fire engines and police cars was a highlight for the children, she said.

Five-year-old Maiah Marchioni​ was sitting transfixed at the window of the play centre's staff room window.

The window provided one of the best vantage points of the carnage.

DOMINICO ZAPATA/FAIRFAX NZ Emergency services manage the crash scene.

"There was dirt flying everywhere," Maiah said.

She was on school holiday and spending time with her grandmother, who works at the childcare centre.

Seeing a bus crash like that was exciting, Maiah said.

DOMINICO ZAPATA/FAIRFAX NZ The bus snapped these retaining poles like match sticks.

She was looking forward to telling her siblings and classmates about it.

"Yeah, I'm going to tell everyone," she said, nodding her head vigorously.

Pavlovich Transport runs the Orbiter buses in Hamilton and Auckland.

The driver has been pulled from service, chief executive Bernard Pavlovich said.

Pavlovich believes police, who are investigating the failure, won't find any mechanical fault with the bus, though he wouldn't be drawn into saying what he thought went wrong.

"Some days it doesn't go quite according to plan."

The bus appeared to have stopped short of the hospital's Percival Flats building, which houses about 15 administration staffers, Waikato District Health Board spokeswoman Lydia Aydon said.

But they will be working from another building for a while, just in case the bus moves.