The prosecution has failed to prove a police sergeant accused of assaulting five prisoners was responsible for causing an injury to a man which caused him to lose a testicle, a court was told today.

Martin James Folan was initially charged with five counts of assault and one of injuring with intent to injure, but the latter charge was changed to one of assault in Auckland District Court today.

Defence counsel Richard Earwaker argued, and prosecutor Mark Zarifeh did not disagree, that the Crown had not been able to provide enough evidence to prove that an injury to the testicle of prisoner Joseph McGee was inflicted by Folan, who was accused of kneeing him in the groin while in custody.

Mr McGee previously told the court he had been assaulted by six to eight men outside a west Auckland bar, which a medical expert could not exclude as a possible cause of the injury.

Mr McGee also made reference in a police statement to being assaulted on another occasion, which may have caused the injury.

The defence started delivering its case this afternoon, with Folan's lawyer Richard Earwaker telling the jury the credibility and reliability of witnesses must be carefully considered.

"Already you (the jury) have seen the effect of cross examination - the Crown case has now changed because the evidence could not support the charge.

"Trials are fluid things - things change - so it is important for you to keep an open mind."

The first allegation is that Folan elbowed a teenager in the face in the back of his police car but the defendant would argue any physical contact with the prisoner was accidental, Mr Earwaker said.

Folan is also alleged to have kicked a man in the groin when he went into his cell to restrain him, but Mr Earwaker said this was merely a defensive push with his foot after the man rushed at him.

The third and fourth charges relate to Folan smashing a prisoner's head into a concrete flaw, which the defendant denied, claiming any force he did use was necessary.

The allegation that he grabbed a prisoner around the neck and slammed him against a wall was also incorrect, Mr Earwaker said. Folan had merely grabbed the abusive, aggressive man on the jaw and told him to calm down.

He then told the prisoner something like, "we're both too old and fat to be fighting" and they shook hands, Mr Earwaker said.

He said any force he had used on the man who lost the testicle was reasonable in the circumstances.

Folan also gave evidence for the first time this afternoon.

He spoke with a thick Scottish accent - he came to New Zealand from Glasgow in 1996 - and was softly spoken and composed in the witness box.

He will continue giving evidence tomorrow.