A woman on a parked bus was left shaken when the vehicle rolled down a hill and crashed after its driver hopped off for a "comfort stop".

Emergency crews were called to Pembroke St, in Hamilton, after the bus crashed down a bank just after 8.30am.

The Orbiter bus ended up on Waikato Hospital grounds, narrowly missing the Percival Flats building, used by administration staff.

A spokeswoman for the Waikato Regional Council, which contracts Pavlovich Coachlines to operate its Orbiter buses, said they had spoken with the woman, who was the sole passenger at the time.

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The spokeswoman said Pavlovich, as well as the police, were investigating. WorkSafe NZ was also looking into the accident.

It is thought the driver, who was not on the bus when it started to roll, had stopped at the area for a toilet break.

"The bus has been removed and the road has been reopened. We understand that the driver had actually gotten off the bus after parking on Pembroke St and prior to the bus going down the hill and over the bank,'' the spokeswoman said.

"There was only one passenger on the bus and the driver had gotten off. It was while he was off the bus that it started to roll down the hill and over the bank.

"We have spoken with [the passenger]. She wasn't injured - obviously shaken, but not injured.''