The level of violence in prisons is as alarming as fight clubs in the former Serco-run Mt Eden Corrections Facility, Chief Ombudsman Peter Boshier says.

Mr Boshier released a scathing report into Christchurch Men's Prison after an unannounced inspection earlier this year. Almost half the prisoners who filled out a survey said they had been assaulted.

It is the third inspection report to be published under a United Nations convention to ensure those imprisoned are treated humanely.

The report identified three areas of real concern - the levels of violence, treatment of mentally unwell prisoners and the prison's failure to implement recommendations made in 2012.

"The first is the levels of violence and what that leads to and the fact that it's just unacceptable in a civilised country like New Zealand," Mr Boshier said.

Investigators checked with staff and looked at reports and medical records in addition to surveying inmates, Mr Boshier told Morning Report.

"There is a high level of violence, there's no doubt about that."

Much of it went unreported because inmates feared the consequences, he said. "I'm justified in saying this because we know that under our noses in the Auckland maximum security prison fight clubs developed when under the control of Serco and no-one knew about it. No one was reporting it, no-one was recording it - in fact people were almost turning a blind eye."

The UK-based company had its contract to run Mt Eden Corrections Facility cancelled after it was riddled with allegations of fight clubs, poor supervision and understaffing.

The government took control of the prison's management in 2015 after the allegations were made public.

But now, rates of violence in government-run prisons had reached an alarming rate, Mr Boshier said.

"When you combine high levels of violence with prisoners who are very mentally unwell at times and not being treated properly that leads to a very volatile mix."

However, he added that it was not surprising as the prison population continued to balloon and despite the introduction of double bunking, there was still the same level of staffing.

Prisoners were more likely to lash out if they were in crowded spaces, especially those who are mentally unwell. "The prison population is just ballooning ... stresses and strains develop in a way that wouldn't if people have dignity and space."

Corrections: More prisoners with mental health issues

Corrections' chief custodial officer Neil Beale said the department's 2016/17 annual report showed serious assaults on prisoners by prisoners is on the decline.

"That's not to say we're not concerned about violence - of course we are. But 75 percent of the prisoners we are currently looking after either have a current or previous conviction for violence.

"They're violent on the outside and when they come into prison some of them continue to be violent with each other.

"Whether somebody's arguing [violence] is going up or down it doesn't sway us from dealing with the issues at hand.

"Yesterday at Mt Eden prison I saw the prison director and his team deal with some very, very difficult challenging people - one prisoner who'd been involved in assaulting people the week before.

"These guys were challenged, they were held accountable, it was done in a very fair, very transparent, very honest way."

One inmate had assaulted a prisoner because he thought someone had stolen a phone card. "When these things happen our staff have to get involved in that, put themselves at risk.. to get involved between two prisoners who are assaulting each other for sometimes the most basic issues."

There had been a 20 percent increase in the prison population in the last two years - most of that in the remand population, he said.

Mr Beale said about 90 percent of prisoners have or had serious mental health conditions and prison officers are working to keep them safe and care for them compassionately.

He said Corrections was investing an extra $11.6 million over four years in dealing with at-risk prisoners with mental health issues.

Earlier this year, Mr Boshier said he was considering an investigation into the treatment of high-risk mentally ill prisoners because of the rates of self-harm and the impact of those prisoners - especially when they're released and pose a risk to the public.

He added that he had approached Corrections Minister Kelvin Davis about his concerns.

"Unless something occurs we will initiate an investigation ourselves into what we're beginning to find is a volatile and unsatisfactory prison system.

"The concern is that prisons aren't designated as hospitals and therefore can't compulsorily treat really ill prisoners from the mental health perspective."

He said prisons were not permitted to give inmates anti-psychotic medication, and the entry requirements to a forensic facility - like the Mason Clinic in Auckland - were very high as there were limited beds.

Mr Boshier said a "significant amount" of prisoners had acute and "worrisome" mental health issues.

"If we don't come to grips with treating prisoners with high mental health needs, that's going to come back and bite us, badly, when those prisoners are released," he told Morning Report today.

The office of the ombudsman is responsible for inspecting prisons so that they meet the minimum United Nations standards.

"We will increasingly, publicly draw attention to situations which we think are unsatisfactory and recommendations we've made," Mr Boshier said.

"If those aren't implemented, that to me is unacceptable and we will continue more stridently to draw attention to where we believe there has been an unsatisfactory response."