Some prisoners housed in the Corrections' Extreme Risk Unit have spent over 900 days in solitary confinement - a breach of international human rights, where the maximum is 15 days.
It is the first time the unit, which was set up in 2019 in the wake of the Christchurch terror attacks, has been inspected by the Office of the Inspectorate.
PERU, at Auckland Prison, houses prisoners considered by Corrections to present a high ongoing level of risk, including some who pose a very high risk of violence and some who are involved in transnational organised crime.
There were currently 13 people being managed in the PERU.
Chief Inspector Janis Adair acknowledged the prisoners needed additional measures to be managed safely.
"However, despite this, we found conditions in the PERU to be overly and unnecessarily restrictive," she said.
The Inspectorate found on average, the 13 men who were currently housed in the unit spent 632 days there.
Five were there for more than 900 days and two more than 800 days.
Not all have been convicted of serious offending: some of them are on remand, awaiting trial.
"None of the men were mixing with any other prisoners, and there were very few interventions that offered meaningful human interaction or constructive activities. Some of the men had spent months or years in these conditions, which likely amounted to prolonged solitary confinement," said Adair.
It also found many prisoners did not know what they could do, if anything, to progress out of the PERU and into a less restrictive regime.
"We found the isolation and hopelessness experienced by the men was raised as a concern by mental health clinicians," she said.
Emma Priest, a criminal barrister from Augusta Chambers who represents a number of men in the facility, and who was a member of the Parole and Prisoner Rights Committee, described it as a "super-maxi" unit and a "prison within a prison".
"The men live their lives in a 9 sq metre cell. They live in almost complete isolation. Most access a small yard attached to their cell with little sunlight or opportunity for meaningful physical activity," she said.
"Rehabilitation services are extremely limited. Non-contact visits only are permitted. It is causing enormous psychological harm."
The men would always conduct their interactions from non-contact booths and most were handcuffed, even when speaking with their lawyers, she said.
"Any perception or any suggestion of there being a significant risk of harm to either another prisoner or to prison guards, in my view, must be overstated, given that context."
She would be surprised if they were getting their mandatory two hours of meaningful contact from staff or educators, said Priest.
She said it would vary between prisoners, and some might spend time with educators or video call their family, "but I would be very surprised if Corrections were ensuring that every single prisoner had - I think it's two hours a day that's required under the Mandela rules - where they were having meaningful social interactions".
Adair said she expected Corrections to take steps to offer more meaningful human interaction and constructive activities to these men. She also expected Corrections to implement a robust assurance framework to provide safeguards to the PERU decision-makers and the men.
"I hope this report and its findings provides Corrections with important insights to consider how it might safely and securely manage those prisoners it considers pose the most significant risk, while giving proper regard for the impact of such a highly restrictive regime that separates individuals from others for prolonged periods of time."
Prisoners held in prolonged solitary confinement - Inspectorate
Safety was a priority but they were dealing with inmates who were the highest risk, with convictions for terrorism and violent extremism among other things, Commissioner of Custodial Services at Corrections Leigh Marsh said.
"So there's a very tight regime as to how they're managed."
They had regular interactions with staff, however, there were limits on how much they were allowed to mix with each other.
The inmates engaged at times with chaplaincy, religious, educational and psychological services.
"What is being called out here is they're not interacting with each other."
Some contact bewteen prisoners could not go ahead because they were a risk to each other, perhaps for ideological reasons, Marsh said.
The unit was only five years old and while there had been a lot to learn, he was confident it had staff with the right skills to run it.
The announced inspection took place in July 2023. The report did not make recommendations, but included 12 findings across a range of areas including placement decisions and pathways, health and wellbeing, and segregation directions.
Corrections said the findings would inform the work programme to ensure continuous improvement in the unit.
It also said a number of improvements had been made since the inspection took place.
In a letter written to Adair on 12 June , commissioner of custodial services Leigh March and deputy chief executive pae ora Dr Jaunita Ryan said the PERU was managed differently to any other prison unit.
"The PERU was established in recognition that there are relatively small subsets of people in prison requiring additional measures in order to be safety managed."
It said the prisoners pose an ongoing risk of serious violence and some may desire to influence others to engage in serious crime.
"These risks may include violent extremism and terrorism," it said.