The police watchdog found officers were justified in arresting the drunk man, but the actions of one cop after the arrest were anything but.

A police officer used gratuitous force against a drunk man, repeatedly punching him before pushing his head into a concrete wall, a watchdog has found

Meanwhile, the cop's fellow officer earned praise from a judge for swiftly reporting the series of attacks after the arrest.

The disgraced officer was discharged without conviction, ordered to pay reparation to his victim and has now left the police.

On Thursday, the Independent Police Conduct Authority (IPCA) released the findings of its investigation into the 2018 incident in central Auckland.

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Police were called on August 24 of that year to reports of a drunk man in the street behaving in a disorderly manner, according to a summary of the incident, where all officers are unnamed.

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When "Officer A" went to make an arrest, the drunk man punched him in the face.

A struggle ensued, involving four officers, and the man was arrested.

While the force used during the arrest was acceptable, what happened next was far from it, the IPCA found.

ANDY JACKSON/STUFF A police officer has earned the praise of a judge for dobbing in their colleague for repeated acts of gratuitous violence.

Officer A tried to hoist the man to his feet by yanking the man's arm inappropriately.

After the drunk man was walked to a police van, the officer then forced the man's head into the side of the vehicle and kneed him in the face.

"Officer A then followed the man into the van and punched him repeatedly."

Seeing this act of repeated violence, and hearing Officer A threatening the man, "Officer B" pulled Officer A out of the van.

But the drunk man's ordeal was far from over.

Upon arriving at the Auckland Custody Unit, Officer A again climbed into the police van and punched the man several more times.

He force-marched the man to a search area, pushed his head into a concrete block wall, then searched him aggressively and unleashed one more punch.

Officer B swiftly reported the misconduct and police launched a criminal investigation.

Officer A was charged with four common assaults along with threatening to kill, and was stood down while awaiting trial.

He pleaded guilty to two amended charges of common assault shortly before the trial date and the other charges were withdrawn.

Officer A resigned from police before sentencing in January 2020.

He was discharged without conviction and ordered to pay reparation to his victim.

The IPCA was satisfied he used excessive force as retribution for the man having punched him during the arrest.

IPCA chair Judge Colin Doherty acknowledged the swift police response to the incident.

"Gratuitous violence of this nature on the part of anyone, let alone a police officer, is never acceptable.

"It is unlikely this incident would have come to light had Officer B not reported it to his superiors. I commend him for doing so."

Superintendent Karyn Malthus, Auckland City district commander, said police agreed with the IPCA's findings.

"Police hold our staff to very high standards, as the public would rightly expect, and this former officer's actions were very disappointing and not in line with the values of our organisation.

"The nature of policing is unpredictable and everyday our staff are faced with difficult, and often volatile, situations.

"However I fully acknowledge that on this occasion the officer 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."

Her prepared statement pointed out the IPCA report found that Police were justified in arresting the disorderly man.