One of New Zealand's top police officers helped arrest an alleged fleeing armed car-jacker this morning while off-duty and heading to the beach.

Canterbury district commander Superintendent John Price was driving in his unmarked car, complete with lights, sirens and police radio, to Sumner beach where he planned to walk his dog.

But while driving through Christchurch at around 9.30am, the radio started chattering about a man who'd just allegedly kidnapped a driver at gunpoint.

The offender, allegedly armed with a pistol, drove the victim from Hawdon St in Sydenham around the city before ending up at an Opawa service station.

The victim managed to escape and alert staff who phoned police.

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Reports of a fleeing gunman in a blue Holden Commodore came over police comms and the off-duty Price realised he was nearby.

"I was looking out for the car and it actually came into sight," Price said.

He joined a senior sergeant in a marked patrol car in pursuit of the alleged kidnapper.

On Mairehau Rd in Shirley, the driver tried to avoid road spikes and ended up in a ditch, Price said.

"Myself and the senior sergeant then arrested the offender," said Price.

The alleged offender is in police custody and will be charged with kidnapping, unlawful taking of a motor vehicle, dangerous driving, and failing to stop, Detective Sergeant Michael Freeman said. He will appear at Christchurch District Court on Monday.

Freeman said the victim is "safe and unharmed" and that no members of the public were hurt.

For 53-year-old Price, who took over the reins as Canterbury's district commander two years, it was all in a day's work.

He often volunteers for front-line policing, and much to his wife's irritation, often works while off-duty.

"I do a number of arrests every year and pull people over after hours," Price told the Herald today, taking a break from a mountainbike ride.

"My wife gets a little bit sick and tired of it, especially if we're going to the movies or something and we're going to be late because someone hasn't put their seatbelt on. But I'd rather take a couple of minutes to save a life than have something worse occur."

He doesn't mind rolling his sleeves up. When police staff were searching Kate Valley landfill site in North Canterbury for clues relating to the disturbing disappearance of Christchurch builder Michael McGrath, Price spent a day sifting through piles of reeking rubbish.

"Even though you're running a big district, at the end of the day you're still a police officer," said the senior policeman with 30 years' experience.

"I won't ask my staff to do anything that I wouldn't do myself. It's all part of being in a team."

Price was glad to help out this morning and see that the alleged offender, his staff, and the public, were all safe.

"When you get a win like that, it's a good result for everyone," he said.

"I'm just one member of a fantastic police team that we have and I honour them every day for what they do. So if I can help out in a little way to achieve a result I will do that."