The teenager's actions were meant to be provocative, but Constable Marcus Dominey's reaction was found to be excessively gratuitous.

A policeman repeatedly punched a youth who spat at him, even as the teenager cowered with his head between his legs.

Constable Marcus John Dominey has escaped a conviction for his actions, which a judge called a display of "testosterone".

However, he has lost his name suppression and the assault is part of a police employment investigation.

Oisin Duke Constable Paul Mackay was a "hero" for reporting his colleague's behaviour, the judge said.

The 38-year-old Timaru officer was found to have assaulted the youth, leaving him with black eyes and a lump on his head. The youth had taunted Dominey by speaking of various diseases as he spat.

Dominey's name suppression lifted on Friday, after a hearing in the Christchurch District Court.

Judge Raoul Neave, who heard the assault case, hailed the colleague who reported Dominey to his seniors, Constable Paul Mackay, as the "hero of the situation".

The judge had found the charge against Dominey proved at a defended hearing in the Ashburton District Court.

The youth had spat at him while they were in the car together in June 2014.

Constable Dominey said he punched the youth because he feared he could get an infectious disease if spittle struck his eyeball.

However, Mackay – who was driving the patrol car – gave evidence that the punching continued while the youth's head was between his knees.

There was ample ability to control the complainant without any further force and the subsequent punches were unreasonable. It was likely that at his point the officer had simply become angry, the judge said.

At Friday's hearing, defence counsel Pip Hall QC argued Dominey should be discharged without conviction because of the likelihood Dominey would lose his job as a police officer.

There was also concern about the effects on his ability to travel to the United States and Canada.

Judge Neave said the police disciplinary inquiry would go ahead whether or not there was a conviction, which would increase the risk of Dominey losing his job.

In a statement released after Dominey's sentencing, district commander for the Canterbury District, superintendent John Price, said police took their conduct very seriously.

Price said Dominey was stood down from his role early this year.

The judge noted Dominey had served his country and his community as a soldier and a policeman.

As a police officer he was in a privileged position, but he was expected to behave with sensitivity and restraint.

The youth's actions had been unpleasant, aggressive and risky and were intended to be provocative, but the officer's response was gratuitously violent.

"This very quickly became an exercise in displays of testosterone," said the judge.

He noted that if a spit-resistant mask had been available at the time, Dominey would have been able to adopt different procedures.

Judge Neave granted a discharge without conviction, but ordered Dominey to pay court costs of $130 and make a $500 donation to the local women's refuge.

The police had wanted Mackay's name suppressed, in case of any reaction by other police staff and the local community, but Judge Neave refused.

The constable had behaved "with the utmost probity" within the standards expected by the police.

"He should be praised," Neave said.

"His conduct could not be more commendable ... he's the hero of this situation.

"If there are elements in the police force and the local community who think otherwise, they should have a long hard look at themselves, because they are a disgrace."

Mackay declined to comment.

The mother of Dominey's victim said her son was "not an angel" but did not deserve a beating.

The Timaru woman, whose name is suppressed, says she now takes "a very dim view" of the police and has lodged two complaints with the Independent Police Conduct Authority since her son's beating.

Her son, who is remanded in custody on charges unrelated to the incident, received black eyes and a squash ball-sized lump on his head.

Dominey's discharge without conviction left her "speechless" with disappointment.

"What a waste of time," she said.

A New Zealand Police Association spokeswoman said the association was unable to comment on individual cases.

An article in the April edition of the association's newsletter, Police News, stated "Cops get used to verbal abuse, and they expect to get into the occasional scrap that can lead to cuts and bruises."

"Most of them would probably prefer that to being spat at – especially if a nasty gob of saliva or blood gets anywhere near their face."

The article stated the association regularly received reports of assaults on officers involving spitting, "often from the confines of the back of the patrol car".

It described spitting as "a very provocative act".

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