A lawnmower man was bitten by a police dog while helping officers chase the 14-year-old driver of a stolen car.

The contractor was one of three bystanders who tried to help after a police pursuit ended in a three-car pile-up in Rosebank Rd in the West Auckland suburb of Avondale about 12.30pm yesterday.

Home businessman Gary Worsley heard police cars "go whistling past" and ran to grab his police scanner.

"They were screaming [on the radio], 'Throw the spikes, throw the spikes'," he told the Herald.

When he emerged from his home - near the intersection of Victor St - an officer leaped out of a patrol car and pulled out a suitcase with road spikes. The teenage driver swerved to miss them - narrowly avoiding the officer - and ploughed straight into a small brown car.

"He kept his foot to the floor and pushed the car out of the way," Mr Worsley said. Two other cars - believed to be a white truck and a Subaru wagon - were damaged in the crash that began when police were alerted to "suspicious behaviour" at a Mt Roskill house.

Police spokeswoman Noreen Hegarty said police pursued the stolen Subaru Forester for two minutes before the air support unit took over. The Auckland serious crash unit attended and is investigating.

Mr Worsley said the teenager saw that he was blocked in and jumped from the car, sprinting down a back driveway. He and two lawnmowing contractors working on a nearby rented house took off after the youth and watched as he jumped a fence.

The three were joined by a dog handler and Mr Worsley was horrified when the German shepherd bit one of the men.

"The three of us were standing on the fence when the dog latched on to the guy right next to me, which scared me," he told the Herald.

Auckland dog section Inspector Mark Hall said both dog and handler were immediately stood down.

"It's a most unfortunate occurrence," he said. "The dog was on a lead so that's how short the distance was ... We're not talking about 20m away, this is on the end of a tracking harness, which from memory are about 5m long."

Mr Worsley said a paramedic bandaged the man's left leg - which was saved by a gumboot - and he was taken to Auckland City Hospital for stitches and a tetanus shot. A hospital spokesman said the man was discharged yesterday afternoon.

Last night the 14-year-old driver and his 15-year-old passenger were being spoken to by detectives, with charges likely.

The dog bite follows an incident in February when 63-year-old Auckland man Brett Abraham was mistakenly set upon by a police dog while chasing away an intruder. The dog was put through rigorous testing but had proved he was fit for duty, Mr Hall said.

He said the dog involved in the Rosebank Rd incident would also be subject to strict testing, in simulated similar conditions, before being allowed back on duty.