The Independent Police Conduct Authority (IPCA) has found the police should not have tried to ram a car off the road in Christchurch shortly before it was involved in a crash.
In November 2014, police tried to block a BMW leaving Christchurch Airport, which was carrying a suspected drug courier.
The driver of the BMW rammed a police car and drove into an officer before fleeing.
The police pursued the BMW for six minutes and tried to force it off the road.
Soon after, on Yaldhurst Road, the BMW crashed into another car, seriously injuring the three elderly people in it.
The two people in the BMW were also seriously hurt.
The IPCA investigation found the police were justified in pursuing the car, but should have abandoned the pursuit when it was confirmed they knew the identity of the driver.
The authority also found the police were not justified in attempting to forcefully push the BMW off the road.