Grant Torrance didn't hesitate when he saw a police officer in trouble.
The holder of a 3rd degree black belt in karate, the 54-year-old senior manager for Hawkins Construction spotted a police officer backing away during a violent confrontation with a young man on Main North Rd in Papanui last week.
"Out of the corner of my eye I saw the policeman back-pedalling with this guy all over him," Mr Torrance said.
He stopped, leapt out and asked the officer if he needed a hand.
Constable Grant Stewart gave the okay and Mr Torrance used his karate skills to help subdue the aggressive offender.
"There was a bit of a scuffle," he said. "I locked him up and basically pinned him to the ground so he couldn't do much. He was pretty well contained."
Police said Constable Stewart had used pepper spray in an unsuccessful attempt to subdue the man, but had been unable to use his radio to call for back-up because the handset had been dislodged during the tussle.
"He was a big guy about 6ft and solidly built and he was cranking ... really wound up," Mr Torrance said. "You could see the pepper spraycan on the ground. I didn't think about what I was going to do, I just wanted the guy to stop."
Mr Torrance said he'd done something similar about 15 years ago when he stepped in between two people fighting, but was modest about his latest success.
"I was just Johnny on the spot. I stopped and gave a hand without thinking too much. It was the right thing to do.
"It's not the sort of thing you expect on the street at 7.15 in the morning."
He was uninjured in the fracas, but admitted "feeling pretty sore" the following day.
Mr Torrance said he was "delighted" to receive a letter from police a few days later thanking him for his efforts.
"It was great, but they didn't need to do that."
Inspector Derek Erasmus said police were "very appreciative" of Mr Torrance's help.
A 21-year-old was charged with disorderly behaviour, resisting arrest. Assaulting a police officer and possessing a knife.