A man lost a testicle because he was beaten by up to eight men, not because a police officer allegedly kneed him in the groin, a court has been told.

Sergeant Martin James Folan has pleaded not guilty in Auckland District Court to six charges of assault and one of injuring with intent to injure.

Giving evidence today, Joseph Hannam McGee said he was set upon by six to eight men outside a west Auckland bar in January 2010 and beaten so severely he lost consciousness and all memory of subsequent events.

Mr McGee was arrested for disorderly behaviour and resisting arrest after the incident and taken to Henderson police station, where he felt "some of the worst pain I've ever had" in his groin.

He said his testicle was swollen bigger than a tennis ball and he could not walk.

The doctor told him "he couldn't save it because it was literally like potato mash".

The Crown alleged it was Folan who inflicted the injury and was expected to present witness and video evidence to support this but defence counsel Richard Earwaker said it was a result of the beating.

Mr McGee had told the court he had only been punched and kicked in the head and upper body during the beating, contrary to an earlier statement in which he said he was kicked in the genitals.

"That's something you've thought of later, isn't it? Because that's what this man's on trial for," Mr Earwaker said.

Another of Folan's alleged victims, Minora Kea, gave evidence today claiming the officer assaulted him after he was arrested in December 2009.

Minora Kea was arrested by police after he had been drinking and was attacked by some men and hit over the head with a blunt object in December, 2009, the court heard today.

He said after being led off to the cells with a sore head, he was let out and then throttled by Folan in the watch house area of the Henderson police station.

"He came up and grabbed my neck and pushed his thumb in, I was pushed against the wall. It hurt and I couldn't speak properly."

A DVD played to the jury showed Folan approach Kea and put his hands around Kea's neck.

"I didn't expect him to do that to me. I feared for my life," Kea told the court.

He said he was unable to talk after the alleged assault and suffered ongoing problems because of the resulting neck injury.

Mr Earwaker questioned the validity of the Mr Kea's claims, pointing out he had made no mention of the neck injury until a year later.

Earlier, the court heard how Samuel Verdonk was allegedly kicked in the groin by Folan after repeatedly banging his head against his cell wall.

Verdonk was intoxicated and "flipping out" when police arrived at his ex-partner's home in November 2009, the court heard today.

"I just wanted to let loose."

He told the court that he reached around a police officer's belt searching for a gun because he wanted to shoot himself or others.

Verdonk remembers very little of being arrested or his time in a police cell. He said he does remember a "nice pretty police woman" waking him up in the morning.

He said once he got home and took a shower, he noticed pain in his groin.

Under cross examination from Folan's lawyer, Richard Earwaker, he confirmed that he had not been spoken to by police until four months after the incident.

Yesterday, Crown prosecutor Mark Zarifeh said the court will hear how Folan asked a constable to swing Verdonk's cell door open. Folan is alleged to have rushed into the cell and kicked Verdonk in the groin.

Earlier, the court heard how a 16-year-old boy arrived home in the back of a police car, yelling that he had been hit in the face by a Sergeant while handcuffed.

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

Mr Campbell said Folan accused him and his friends of smashing letterboxes.

Mr Campbell denied any involvement, but confirmed 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."

The teen's father, James Campbell, said his son had never been in trouble with the police before.

He said he and his wife arrived home from dinner to find their son handcuffed in the back of a police car.

Folan told them that Finn had been "lippy" and may tell the court that he had been elbowed in the face but that if there had been any connection, it was an accident.

Mr Campbell said his son was distraught.

"As soon as the car door was open, Finn was shouting: 'He elbowed me in the face'."

Mr Campbell said it took about two hours to calm his son down.

"It took me two days to convince him - it seems strange, I wish I hadn't - but to convince him not to lay a complaint."

Mr Campbell said there were no marks on his son's cheek.