A woman injured in a "freak accident" when a unoccupied car rolled into her leg at a hot rod event is believed to be the wife of a car club member.

The parked car's handbrake came undone before it rolled into the woman in the car park of The Hangar bar on Central Park Drive in Henderson about 10am.

It is believed the woman was attending a Wild West Sundays hot rod car event at the bar at the time of the accident.

Inspector Andy Brill of police northern communications said the car's handbrake "gave way".

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"The handbrake came undone and it rolled into somebody and damaged their leg."

A police spokeswoman said there was no one in the vehicle at the time of the accident and alcohol was not involved.

The woman was taken to Auckland City Hospital in a serious condition. A hospital spokeswoman said she was now stable.

According to its Facebook page, Wild West Sundays events are for classic car enthusiasts to meet and look at cars over coffee and breakfast.

In a Facebook post after today's incident, event organiser Vince Lettice described it as a "freak accident".

"We would just like to say that this was a terrible accident that happened at the Hangar this morning, our thoughts are with Scotty's wife, and hoping [for] a speedier recovery from this freak accident."

Mr Lettice said he had helped to remove the car, which had rolled into the woman and another car.

Incidents like today's could happen at even the best run and organised events, he said.

Police said they had concluded a scene examination.

Staff at The Hangar said they had been advised by management not to comment on the incident.