The police watchdog has found a Tauranga officer used excessive and unlawful force when he twisted a 14-year-old boy's arm up his back and choked him.
Two St John's staff complained to police after seeing the incident in the back of an ambulance in August last year.
The boy had cut his wrists while under the influence of alcohol and drugs and had twice run away from hospital.
The second time he was found, one of the ambulance officers said an officer almost dislocated the small mentally-ill boy's arm and put his hand around his neck until he changed colour.
The Independent Police Conduct Authority has found the officer was entitled to use some force, but what he did use was excessive and contravened the law.
It said the officer was not prosecuted because of conflicting evidence.
A Bay of Plenty police spokesperson said the officer should have taken more time to try to de-escalate the situation and appropriate action had been taken against him.
However, he would not be prosecuted because of the conflicting evidence, the spokesperson said.