Urgent action is needed to stop violence and intimidation at Whanganui Prison, Chief Ombudsman Peter Boshier says.
Inspectors made an unannounced inspection at the prison in February and found there was a high number of violent incidents at the prison each month and these had been increasing.
"I consider there is a clear and urgent need for the prison to address the levels of violence and intimidation," Boshier said.
Two of the prison's units had the highest number of recorded assaults of all the Department of Corrections' lower north region facilities.
Boshier said levels of violence and intimidation are likely to be even higher because surveys suggested 71 per cent would not report incidents.
Common reasons for not doing so included a perception that gangs are in control and that reporting would cause them even more difficulties.
"Prisoners fear for their safety so they don't report the stand-over tactics or bullying they experience from other inmates, yet they tell my inspectors that these are common occurrences," Boshier said.
"My inspectors also observed that incidents, like fighting, sparring and unexplained injuries, have increased since the last inspections."
Boshier said the prison did not have an active gang management strategy in place, although the Prison Director confirmed he was in the process of developing one.
This was surprising, Boshier said, given more than forty per cent of the prisoners were either gang members or associates.
"My Inspectors concluded that the pervasive influence of gangs at the prison was having a detrimental effect on both prisoners and staff."
Meanwhile, Boshier said inspectors found generally positive interactions between staff and prisoners.
"Prison staff described the level of co-operation between groups as 'better than in some other places'.
"This was valued as pressures resulting from increased prisoner numbers were identified as raising stress and fatigue issues for staff.
"We found case managers provided a timely and satisfactory level of service to their prisoners and prisoner admissions were carried out in a calm and measured manner."
Accommodation was generally clean and well maintained in all but two complexes, inspectors found.
The ventilation in both high and low-security units was acknowledged as inadequate.
Overall, health services were considered fit for purpose and staff believed the addition of mental health clinicians would provide them with more support.
However, there was little evidence found of care planning for patients with complex, long-term conditions. The report was given to Parliament on Tuesday.