A custody officer used excessive force when he threw a man into a wall while trying to restrain him, the police watchdog has found.
The Independent Police Conduct Authority said the man had been arrested for breaching his bail and was intoxicated when taken to the Counties Manukau station two years ago.
The man was described by some police staff as "fooling around" but the custody officer interpreted his behaviour as aggression.
The authority said in an attempt to restrain the man, the officer flung him into a wall. The man's head hit the wall with some force and CCTV footage showed him sliding down the wall to the floor.
Though he appeared to be unconscious, the custody officer and another continued restraining the man.
IPCA general manager Warren Young said the authority considered unnecessary and excessive force to have been used.
"The authority thinks it's important that those who are in custody are treated appropriately and with restraint.
"Because we have found that force is excessive there may potentially be criminal or employment consequences for the staff member, but the authority has no juristiction over that, that is a matter for the police."
Mr Young said the authority found the officers also failed to tell a doctor the man may have suffered a head injury.
"We have also criticised the officers in the custody unit for not arranging for the man to have medical attention, and we have recommended that the police ensure that in instances like this medical officers attending the unit have access to CCTV footage," Mr Young said.
"We think it's important so that doctors can assess for themselves whether there's likely to be an injury that needs attention. In this particular case the doctor was not told that the man had hit his head on the wall and therefore did not assess whether he had a head injury."
The IPCA report said police investigated the incident and the custody officer was placed on restricted duties and charged with injuring with reckless disregard for the safety of others. It said police later withdrew the charge and carried out an employment investigation and the officer was sanctioned.
Police Association president Chris Cahill said the charges were withdrawn because the officer used a maneuver he'd been trained for, an arm bar takedown, which is designed to bring a suspect to the ground without excessive force.
Mr Cahill said the injury was not intentional.
"Two arm bars, one from each officer, were applied, to force him down to the ground. But in doing that in the confined spaces of a cell he's hit his head against the wall which wasn't an intentional maneuver.
"In this case, I wouldn't agree with the findings of the Police Authority around unlawful acts that occurred. We know that the officer was originally charged, the case was reviewed and expert advice sought and the charges were withdrawn because it was felt a prosecution wouldn't be successful because they'd acted within the training guidelines," Mr Cahill said.
"I don't think they really understand the dangers in dealing with prisoners and why officers have to be extra careful. We know that many of them are assaulted and so they do need to take extra care, and that's why a prosecution didn't proceed in this case."
Counties Manukau District Commander Superintendent Jill Rogers said police accepted the authority's findings.
"As police, we acknowledge that we have a duty of care for people that are held in the custody unit," Superintendent Rogers said.
"We set high standards for our police staff and we strive to deliver on those every day. We acknowledge in this case the actions of this officer were excessive and we have taken appropriate action as a result."
She said the custody officer remained in the police, however.
"Police, like any other employer, have privacy obligations to consider and are unable to comment on the outcome of this investigation. However, we can confirm the officer remains a member of NZ Police."