A fleeing kidnapper nabbed by Canterbury's top cop on a day off has today been jailed for three years.

Michael Mamea, 31, hijacked a car with a man sleeping inside early on September 23 last year in the Sydenham area of Christchurch.

He drove to a Waltham house and parked in the driveway, telling his victim he was a gang member but if he stayed in car he would be okay.

Mamea soon returned and drove to a petrol station. He took $40 cash from the victim who was told to fill the tank and Mamea went inside pay for the petrol.

The victim walked into the station acting as if he was going to buy a drink but instead raised the alarm with staff, saying he'd been kidnapped.

Mamea took off and was soon spotted by police and pursued.

At the same time, off-duty Canterbury district commander Superintendent John Price was driving in his unmarked car to Sumner beach to walk his dog.

His police radio started chattering about a fleeing kidnapper and Price realised he was nearby.

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

He joined a senior sergeant in a marked patrol car in pursuit of Mamea.

Mamea drove through two red lights and on Mairehau Rd crashed into a ditch trying to avoid police road spikes.

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

At Christchurch District Court, Mamea admitted charges of kidnapping, unlawfully taking a vehicle, dangerous driving and failing to stop for police.

Defence counsel Josh Lucas said Mamea has done a number of courses in prison and once he's released in November 2020, he wants to work on fishing boats to get away from bad influences.

Lucas said it was a "non-violent kidnapping" that did not involve a weapon, with the victim "lying to police" that he saw a gun and later accepted that he hadn't.

Judge Paul Kellar sentenced Mamea to three years, two months imprisonment.

Price said 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," he said.

"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."