Prison overcrowding means less work on prisoners' serious mental health needs who then become "very dangerous" to the public when released, says Chief Ombudsman Peter Boshier.
"They will strike," he has warned.
Boshier said overcrowding was affecting the ability for the Department of Corrections to provide mental health care, rehabilitation and programmes intended to make prisoners functional citizens.
"I am concerned that with the increasing number of prisoners with high mental health needs, unless they are addressed properly, we are releasing prisoners who are very dangerous to unexpecting members of the public."
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Our prisons are bursting at the seams and Corrections is seeking Government approval for the 3000-bed mega prison in Waikato. In the meantime, it has expanded double-bunking, reopened closed prisons and is getting prebuilt cells shipped in from China.
The expansion of Waikeria Prison has been spurred by repeatedly inaccurate prison population forecasts from the Ministry of Justice which miscalculated the effect of "tough on crime" policies.
The mega prison-plan goes to Cabinet for approval next month but runs contrary to the Government's intent to reduce the prison population and Minister of Justice Andrew Little's intent to focus criminal justice reform on "fixing" inmates.
The current prison population sits around 10,700 and is projected to keep rising as high as 15,000 by 2025.
Boshier said rehabilitation had been affected, along with programmes intended to prepare prisoners for release into the community.
"Where we get concerned … is we look at some cells, which are pretty awful, and you'd wonder what you would produce in a prisoner where you will reduce them to a primal context.
"You wonder how they will emerge in our society where their environment has been minimalistic and primal and not particularly geared to reorienting to the community."
Boshier, who has served as a Family Court judge, said the system meant almost all prisoners would eventually be released into society.
"The concern I want to highlight, partly because of my family court background, is the fact that if we don't treat people with high mental health needs, we don't treat them at our peril."
Corrections says that more than a third of prisoners had mental health issues and around half will have an alcohol or drug dependency.
Boshier said: "If there are those in prison in high-risk units whose lethality is not being addressed, when they are released, they will strike.
"And you can look at the reality of that by seeing what happens - the number of people who commit serious crime who have high, unaddressed, mental health needs."
Boshier said New Zealand was at risk of falling below minimum standards set by New Zealand's agreement to comply with the United Nations Optional Protocol to the Convention. Those standards were also broadly captured by the law under which Corrections operates.
The Office of the Ombudsman released reports in December critical of four of the 18 prisons in New Zealand. Boshier said conditions at those prisons existed "nationwide" and increasing violence was a serious issue.
"The personal space that allows people to work through their emotions isn't there and so you get more violence, you get increased (poor) mental health outcomes because people don't have their private time to work through the thoughts that are going through."
Basic principles of imprisoning people close to their home area were being compromised to make room for people with female prisoners from Auckland now locked up in Wellington and Maori imprisoned hundreds of kilometres from the iwi base.
Boshier said it "stands to reason" that an increase in prisoners, "recycling" prison facilities which had been closed" and double-bunking" meant prisoners "not having the same access to facilities which are so important to try and get their heads right".
In one case highlighted by the Office of the Ombudsman, a prisoner was tied to a bed for most of the day for weeks on end.
"The easiest solution was to tie them to a bed all that time, each successive day. A cause and effect of an increased prison population and the ratio of staff being lessened is a use of methods which are not ideal."
He said he had repeatedly raised concerns but "the actuality of addressing it I have yet to see".
"I won't let this go. I will keep on until it is properly addressed."
Corrections national commissioner Rachel Leota said investment in staff, mental health care and rehabilitation had "improved our standards of delivery in keeping New Zealanders safe".
"We have not and will not breach our statutory obligations."
She also said there were "good plans" to handle the prison population.
Leota said there was a "considerable drop" in serious prisoner-on-prisoner assaults last year despite a "dramatic increase in the prison population" and three-quarters of inmates having violence convictions.