At-risk inmates at Waikeria Prison have been spending up to 26 hours at a time locked in their cells, an inspection report has found.
The report, released by the Corrections inspectorate, also found that the prison population had grown so rapidly that the prison, near Te Awamutu in Waikato, had to re-open high-security units that it had previously shut because they were in such poor condition.
The inspection, carried out in August last year, found that in the prison's at-risk unit, prisoners were routinely kept in their cells for 22 hours each day.
"At times they spent 26 hours in their cell between unlocks," chief inspector Janis Adair wrote.
"Health centre staff expressed concern to us about the regime, and one prisoner said he had become more focused on self-harm thoughts while in the unit, due to the lack of other activities to keep him engaged."
The report raised concerns about Wakeria's high-security facility, which was more than a century old and "in very poor condition".
Corrections had planned to close several of the units and had done so, closing the central unit first, followed by two more in 2015.
"[However], because of rapid growth in the national prison population, these two units were recommissioned in 2017."
Two other units slated for closure had also remained open, the report said.
At the time of the inspection, one in six principal corrections officer jobs and one in three senior officer jobs were vacant, it found.
Chief custodial officer Neil Beales said 30 of the 42 issues raised in the report had been resolved, while the remaining 12 were partially resolved.
However, Mr Beales' response to the report did not say how long prisoners in the at-risk unit were now spending in their cells.
Nor did his response set out whether or not the units that had been recommissioned were still being used.
However, 60 percent of the vacancies at the time of the inspection had now been filled, his statement said.
The inspection report found prisoners were generally positive about the support they received from prison staff.
The prison had a strong focus on work experiemce, including operating one of New Zealand's largest dairy farms.
"There is a strong focus on kaupapa Māori rehabilitation programmes," the report said.
"However, access to programmes and other purposeful activity was limited because of reduced unlock hours, due to staffing pressures."