A double-decker bus crashed on Auckland's Northern Busway after the driver was stung by a bee.

The southbound Northern Express ended up in bushes beside the busway this morning after the driver veered into a sign as he was stung, said Auckland Transport spokesman Mark Hannan.

Hannan said all 15 passengers managed to get out without injury. Apart from the sting, the bus driver was also uninjured.

Passenger Patricia Jay​ told the Herald she was sitting on the bottom deck of the bus at the time of the crash, which occurred just north of the Constellation bus station in Sunnynook.

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"We didn't have a clue what happened, it was just all of a sudden we were on the bus lane, then we were not," she said.

"It was a really kind of scary experience and it would have been awful upstairs."

She said the passengers and driver were lucky the bus did not hit the concrete barrier, but rather came to a stop on a roadside grass strip.

She said the bus appeared to be full, with many people travelling to events for the Masters Games.

"I can assure you there was more than 15 people," she said, refuting official claims of passenger numbers.

She said one passenger "banged his head, but seemed okay" in the crash.

The New Zealand Transport Agency said the crash was clear of lanes on the busway but rubberneckers were causing problems for traffic heading south on the adjacent motorway.

Traffic stretched as far back as Oteha Valley Rd.

Police said passengers managed to get out of the double-decker bus, which ended up tilted into bushes.

The crash happened about 200m from Constellation Drive.

Hannan said the passengers had been transferred to a second bus.

The busway was now open.

He said there was minor damage to the bus, which would be towed away as soon as possible.