Two police sergeants are under investigation after allegations they tried to convince colleagues not to give evidence in the trial of an officer accused of assaulting prisoners.

Martin James Folan, a sergeant based at Henderson in West Auckland, was found not guilty by an Auckland District Court jury of six charges of assault last week.

At the trial, the Crown alleged Mr Folan had acted in anger when he elbowed a teenager in the face, banged a man's head on a police station's concrete floor, kicked a man in the groin or stomach, throttled another man, and kneed a fifth man in the groin.

A number of police officers gave evidence in court - both for and against Mr Folan.

The Weekend Herald has learned two sergeants working in the Waitemata District are under investigation after claims they put pressure on some of the officers set to testify against Mr Folan.

It is understood they were trying to get the officers to change their mind about giving evidence.

Waitemata District Commander Superintendent Bill Searle refused to comment on the details of the investigation.

He would not say whether the men had been stood down, or how the allegations against the sergeants came to his attention.

"As there is an employment-related inquiry in relation to Sergeant Martin Folan in progress, police cannot comment on any matters pertaining to this employment related inquiry."

A senior police source said last night that the latest investigation into the two sergeants was not a good look for the force.

He said Mr Folan's trial was "bad enough".

"But this is out-and-out a bad culture ... I would hold them in more contempt than I would [an officer charged with assaulting prisoners]."

Mr Folan, who has been a policeman for 14 years, was stood down in February last year and has not returned to work since.

He is currently facing an internal disciplinary hearing and is the subject of an Independent Police Complaints Authority inquiry.

At the end of his trial his lawyer Richard Earwaker acknowledged police had a job to investigate their own personnel when complaints were made but the process was "incredibly difficult" for those involved.

"We don't want to get a situation where police officers are afraid to do their job because of who is looking over their shoulder."