A ruling from Independent Police Conduct Authority has found a Christchurch officer used excessive force when handling an aggressive man.
The incident in June 2020 involved a man behaving aggressively when asked to transfer between holding cells.
A violent struggle broke out when the man, who was in the cell for breaching bail conditions, refused to cooperate with the officer's request to move.
One officer stood in the doorway explaining the need to move, while two others stood behind in support.
When he started making advances, the officer in the doorway pulled him down, and struck his head multiple times to distract the man as he attempted to grab his testicles.
Judge Colin Doherty ruled the response was necessary to control the man's behaviour, but were excessive in the purpose of self protection.
"These actions were beyond what was necessary for the officer to protect himself or bring the man under control, and, in my view, were at one stage motivated by anger," he said.
Doherty also said it was extremely concerning that the man wasn't seen by a doctor after receiving blows to the head.
Christchurch police say they're improving processes around transferring aggressive people in custody between cells.