Police officers who forced prisoners to pose for degrading photographs and dealt out excessive force were not reported because of a so-called "blue code of silence" that exists within the police.

An inquiry into South Auckland police has found no evidence of a "sick culture" but has uncovered new allegations of abuse not previously reported because of the code.

Inquiry head Sir David Tompkins yesterday released his report on the findings of a six-month investigation, sparked by the trial of senior officer Anthony Solomona on five assault charges early this year.

The trial was told about a photograph of a prisoner forced to wear a sign saying "I belong to Senior Sergeant Solomona". Another photo, showing an officer wearing a balaclava and holding an axe and a machete, was produced in evidence.

Judge Bruce Davidson found Solomona guilty on one assault charge and commented that in South Auckland "the culture is as sick as the joke".

But Sir David found the photographs and cases of excessive force "did not amount to a culture of excessive violence or inappropriate conduct ... as they were not practices adopted or condoned by a significant number of police in the Counties Manukau Police District".

He said the officers involved had been connected to the Emergency Response Group, which was disbanded in March last year.

Solomona was in charge of a section in the group.

The culture inquiry highlighted 20 cases of police using excessive force in a five-year period.

"These instances related mostly, but not exclusively, to members of the ER Group and senior [non-commissioned officers - sergeants and senior sergeants] with a history of complaints. Although the use of excessive force has been reduced, it has probably not been eliminated."

Sir David said most cases of violence were not reported because of the blue code of silence - a phrase coined in English studies of police culture and referring to a conflict between professional integrity and group loyalty.

A former officer who served in the district for 10 years told the inquiry it was common knowledge that officers used excessive force.

She recalled a prisoner being "dragged by their hair across a road into the gutter".

She did not report the incident, which Sir David said was an example of the code.

"The inquiry team concluded from the responses of former staff that there was limited evidence of minor assaults during the arrest process and that at times improper behaviour was dealt with informally rather than reported, in accordance with the blue code of silence," he said.

A former British officer working in the district said he was interviewing a prisoner in August 2003 when another officer came into the room and "delivered a single punch to the prisoner". He did not report the officer and could no longer identify him.

Another ex-British officer told the inquiry of a "number of officers" acting aggressively during an arrest at a service station in January this year.

"With up to five officers present, there should have been no need for kicks, punches and the use of OC [pepper] spray. The offender was found to have ... puncture wounds on his middle left leg ... and some abrasion to his head, arms and legs.

"The more senior and the controlling officer is an officer with a history of complaints recorded against him. He also worked in the ER Group."

The inquiry was told of other examples of minor assaults on prisoners but overall heard of a positive culture in a demanding work environment.

Counties Manukau District Commander Steve Shortland said any new allegations uncovered by the inquiry would be investigated.

He said police frequently dealt with "raw, unpredictable, human emotion, often fuelled by drugs and alcohol, and a very small number of officers can overreact".

Officers were given training and guidance to deal with such situations in a controlled and professional manner. He did not believe the code was a major issue.

"One of the ways to overcome that is by getting a culture where people put loyalty to the organisation ahead of loyalty to a personal group and ... our senior managers here work hard on pushing an ethical stance and a professional stance."

Acting Police Commissioner Steve Long said the report "puts that notion [of a sick culture] firmly to bed".

The Police Association said it would demand an apology from Judge Davidson for "his clearly unjustified comments on their culture".

"The comments hurt a lot of very good police officers. They really did sting," said association president Greg O'Connor.

"These are the most assaulted, the most beaten, the most complained about. We believe they were almost hung out to dry."

Judiciary communications adviser Neil Billington said Judge Davidson could not respond, because judges could not comment directly about matters outside what they said in their decisions.

The report said there was a higher number of severe assaults on police in Counties Manukau than in other districts, and more than 200 complaints a year laid against officers in the district.

Its police force of 626.5 sworn officers - covering a population of 435,000 people - was under strength and a high proportion of the staff were relatively junior officers.

- additional reporting: Helen Tunnah