Detective Sergeant James Haigh said the stabbing happened near a car wash business at the intersection of Clarence St and Blenheim Rd in Riccarton at 5.40pm on Thursday.
"Two cars were at the red lights on Clarence St when a male has gotten out of one of the vehicles and attacked the victim who was in the other car."
The offender then fled the scene heading towards Riccarton Rd.
Police investigating the attack do not believe the attacker and victim knew each other.
The attacker was alone and the victim was a rear passenger in a red vehicle, police said.
The victim remains in Christchurch Hospital in a critical condition.
Haigh said an investigation is under way and police are appealing to anyone who may have witnessed the attack or has footage of it to contact them.
- Call police on 105 or Crime Stoppers on 0800 555 111 if you an help with the investigation
