The chief ombudsman is calling for urgent action at Whanganui Prison to tackle inmate violence, intimidation and a "pervasive" gang influence.
It follows an unannounced visit in February.
In a written statement, Chief ombudsman Peter Boshier said the number of violent incidents each month was high, and increasing.
"I consider there is a clear and urgent need for the prison to address the levels of violence and intimidation."
Two of the prison's units had the highest number of recorded assaults of all the Department of Corrections' lower north region facilities.
"My inspectors also observed that incidents, like fighting, sparring and unexplained injuries, have increased since the last inspections."
A survey of prisoners showed 71 percent of them would not report incidents, common reasons included a perception that gangs were in control and that reporting would cause more difficulties.
The prison director confirmed he was in the process of developing a gang management strategy.
Mr Boshier said it was surprising the prison did not have such a strategy in place given that more than 40 percent of the prisoners were either gang members or associates.
The statement also said that there was little evidence of care planning for patients with complex, long-term conditions and ventilation in both high and low-security units was acknowledged as inadequate.
Staff believed the addition of mental health clinicians would provide them with more support.
On the other hand, Mr Boshier stated his inspectors found generally positive interactions between staff and prisoners.
"This was valued as pressures resulting from increased prisoner numbers were identified as raising stress and fatigue issues for staff."
Accommodation was generally clean and well-maintained in all but two complexes, the statement read.