Advocates for penal reform say a critical United Nations report on conditions inside New Zealand prisons is an indictment on Government penal policy.
The report, authored by a United Nations committee on torture which visited 35 detention facilities last year, has found many inmates are routinely locked up for 19 hours a day and denied access to rehabilitation and mental health services.
Spokesperson for the Howard League for Penal Reform, Roger Brooking, said the threshold for getting psychiatric treatment was too high.
He said prisoners who are suicidal end up "rotting away" in high-risk units, where they have no contact with other prisoners, no books or television and are checked every 15 minutes, resulting in chronic sleep deprivation.
"In my opinion that constitutes psychological torture."
A former prisoner, who turned his life around through education while in jail, said lack of resources means fewer inmates these days get the chance to break the cycle.
Dr Paul Wood, who went to jail at 18 for murdering his drug dealer and graduated with a PhD in psychology after completing bachelor and master degrees in prison, said those who most need rehabilitation - high-security prisoners - were least likely to get it.
"They're then denied the opportunity for parole because parole is often contingent on having attended such courses. So you can understand why people like that develop a sense of injustice and mistreatment which then gets directed towards society in general."
Dr Wood said the problems exposed in the UN report were much the same as when he first arrived in prison 20 years ago, and he was doubtful the situation would improve "as long as crime and punishment are treated as a political football".
He said while the UN report found no malicious systemic abuse of prisoners, lack of resourcing meant that in some cases, they were not being treated humanely.
The report also notes funding problems, and staff shortages, have hampered the ability of government agencies to carry out their mandate under a UN protocol against torture and cruelty.
The agencies charged with overseeing detention services are the Ombudsmen's Office, the Independent Police Conduct Authority, the Children's Commissioner and the Inspector of Service Penal Establishments.
Director of Rethinking Crime and Punishment, Kim Workman, said he was very disturbed that lack of resourcing had prevented these agencies from doing an adequate job.
He said prisons had to treat inmates with some dignity, "knowing that when you start to treat people like animals, then they behave accordingly".
In a written statement, the Corrections Department said changes have already been made to improve inmate safety and access to mental health services.
Newly built solitary confinement cells in Auckland which the report described as '"in cans" because of their small size, were merely prototypes and have since been replaced by larger cells.