A police officer is on trial after allegedly bashing five prisoners, one of whom had to have a testicle removed.

Martin James Folan, a police sergeant in West Auckland, faces six charges of assault and one of injuring with intent to injure.

He pleaded not guilty in the Auckland District Court yesterday.

Crown prosecutor Mark Zarifeh said the incident that led to the prisoner losing a testicle took place on January 8 last year.

Folan faces a charge of injuring with intent to injure and an alternative charge of assaulting with intent to injure in relation to Joseph McGee.

Mr Zarifeh said Mr McGee was drinking at a West Auckland bar when he got into a fight. An ambulance and the police were called.

Mr Zarifeh said police officers had to restrain Mr McGee while ambulance staff checked him for injuries. He was arrested for disorderly behaviour and taken to the Henderson police station.

Three police officers were helping to process Mr McGee when Folan kneed him "with a good deal of force".

Mr McGee spent the night in a police cell, where he was seen holding his groin in pain. When he got home next morning, he took painkillers, but put off seeing a doctor for three days.

Mr Zarifeh said Mr McGee's swollen left testicle was eventually examined by a doctor and it was removed in hospital.

Police are allowed by law to use reasonable force, and Mr Zarifeh said this would be one of the issues the jury of five men and seven women would have to take into account. But he said Folan's use of force was "gratuitous".

"What really occurred is him losing his temper."

Folan's lawyer, Richard Earwaker, said there would be issues of credibility because none of the complaints was made at the time of the alleged assaults.

He said jury members would also have to ask themselves what happened and, in at least one case, when were the injuries sustained and by whom?

Folan is also alleged to have beaten four other prisoners.

Mr Zarifeh described an incident which happened after Samuel Verdonk was arrested for disorderly behaviour in November, 2009.

Mr Verdonk, who was drunk, had been banging his head against his cell wall when Constable Scott Shearer and Folan decided to intervene.

"Constable Shearer saw the accused lead in with a kick to the groin area."

Jerrilee Tuiva had also been drinking when he was picked up by police for breaching bail on December 8, 2009. He was put in a cell to "cool down" but resisted police, and five officers - including Folan - were needed to control him.

Mr Zarifeh said witnesses saw Folan twice grab Mr Tuiva's head and smash it against the concrete floor.

Minora Kea had also been drinking when he was arrested by police on the same night for disorderly behaviour.

Mr Zarifeh said that the next morning Folan and Mr Kea had an argument.

"He went up to Mr Kea, grabbed him with both hands around the neck and pushed him against the wall."

Finn Campbell told the court that he was approached by Folan after he had been drinking with friends on Halloween in 2009.

Mr Campbell, who was 16 at the time, said Folan accused him and his friends of smashing letterboxes.

Mr Campbell denied any involvement, but admitted to Mr Earwaker, that he got "lippy" with Folan before being put in handcuffs.

His friends were taken home by other police officers, and Mr Campbell was put into Folan's police car.

"He was going to put my seat belt on and I said: 'Why do you have to be a dick about this?"'

Mr Campbell said Folan elbowed him in the face and said, "Don't call me a dick".