A police officer avoided conviction despite repeatedly punching an arrested man and kneeing him in the head.
The Independent Police Conduct Authority said the the officer was repeatedly and gratuitously violent towards the man.
The 43-year-old officer was trying to arrest the man after a disorder callout in August 2018, when the man punched him.
With the help of other officers, the man was detained and taken back to the station.
During the trip, the officer repeatedly punched the man's face, kneed him in the head and shoved his face into a concrete wall, the authority found.
At the police station the officer hit the man with an uppercut to the face, which was captured on CCTV.
A police employee said the officer then threatened to kill the man.
The officer denied threatening or punching the man, despite the footage.
He was charged and pleaded guilty in court to two counts of common assault and one of threatening to kill. Two other charges were withdrawn.
The officer was discharged without conviction in January this year.
An employment investigation followed bu the officer resigned before it was finished.
Auckland City District Commander Superintendent Karyn Malthus agreed that the officer used excessive force after the arrest.
"He acted out of line and abused his position of authority. It is never okay for our officers to act, or react, in a violent manner."