New Plymouth prison guards are under investigation for allegedly giving special treatment to Black Power members on trial for murder.

The Taranaki Daily News understands three prison guards are on stress leave while the investigation is being carried out.

One police officer saw some of the alleged incidents and is helping the Department of Corrections with its inquiries.

Information given to the newspaper involves claims prison guards gave four Black Power members, Matiu Pahau, 22, Mahana Edmonds, 39, Rangi Rick Brown, 40, and Adrian Fenton, 27, special treatment during their murder trial.

It is alleged the guards allowed the accused to have unauthorised visitors, allowed them to make telephone calls, delivered food including fish and chips to the prisoners, and let them smoke in unauthorised areas while they were being held in court cells.

The men were on trial for killing Peri Niwa in New Plymouth on November 15, 2008.

It has been alleged the guards were taking instructions from a senior member of the Black Power gang.

New Plymouth Prison manager Peter Madsen said the matter was subject to an employment investigation.

"This investigation is under way. The department cannot comment any further," Mr Madsen said.

A woman from the Department of Corrections communications section later rang the Taranaki Daily News to say they had conflicting information to that given to the newspaper, but would not say what.

New Plymouth police acting area commander Inspector Blair Telford said the police had not made an official complaint.

"A police officer was a witness to some of the allegations you raise and is assisting corrections with their investigation," Mr Telford said.

The Corrections Department would not say how long the investigation was expected to take, or if the guards were threatened or intimidated by Black Power members.

Pahau was found guilty of Mr Niwa's murder and the other three were found guilty of his manslaughter.

Last month it was revealed misconduct investigations involving prison guards had jumped from 76 in 2004-05 to a high of 286 in 2008-09. During that time, 63 guards were fired for offences including "inappropriate relationships" with inmates.

- © Fairfax NZ News