One prisoner was allegedly locked up for 144 hours straight, some said they ate stale sandwiches, and most inmates reported witnessing an assault.

Results from the Howard League's annual survey of inmates in New Zealand jails paint a "disappointing" picture, the prisoner's rights group says.

However the Department of Corrections has dismissed the survey as "unscientific" and the claims "unsubstantiated".

League spokeswoman Jane Maltby said just 82 prisoners were surveyed, which she agreed was a small number. But she said the point was to ask users what they actually thought of the prison system.

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"Certainly, although we didn't receive that many responses, we receive hundreds of letters throughout the year highlighting these issues."

The results showed Rangipo prison at the central plateau was the most favoured by inmates, while Springhill in the Waikato had the most dismal results.

Chief among the league's concerns following analysis of the feedback was prisoner's access to rehabilitation, adequate food, their safety, and the amount of time they spend outside their cell.

One prisoner had reported being locked up for 144 hours - six days straight - however because the surveys were anonymous it was impossible to investigate further, Maltby said.

Maltby also highlighted prisoners' reports that they had little access to mental health services, or rehabilitation, and prisoners' feedback that those with special dietary requirements were sometimes forgotten.

In response to questions, Corrections chief custodial officer Neil Beales described the survey as "unscientific", highlighting the small number of participants, and pointing out the prison population numbered 10,400.

He said the claims made in the surveys were "unsubstantiated" and Corrections couldn't investigate them unless it was provided with more information.

"However, what we can say is that prisoners have more ways than ever to make a complaint, including to unit staff, the Corrections complaints response desk, the Inspector or the Ombudsman, as well as other external agencies such as the Privacy Commissioner or Health and Disability Commissioner," he said.

Corrections carries out its own quarterly survey of its prisons, giving them reviews ranging from exceptional, to needing improvement.

In response to questions about the alleged lengthy lock up hours, Beale said they were only reserved for dangerous prisoners being held in high risk units.

He said Corrections was doing more than ever to help prisoners with mental health issues, including piloting $14 million worth of initiatives, and that Corrections had a zero tolerance policy to violent.

The food provided to inmates met Ministry guidelines, he said.

"Any specific dietary needs for prisoners are noted and the food is adjusted accordingly."

BY THE NUMBERS

Seventy-two per cent of those surveyed said they felt safe in jail, while 73 per cent said they had seen a prisoner assault another prisoner.

Twenty-three per cent of respondents alleged witnessing a prison officer assault an inmate.

Nearly 80 per cent of those surveyed said prisons had poor access to mental health services.

The majority said they had enough to eat, although Springhill prisoners said portion sizes had decreased.

Fifty-one per cent were unhappy with the quality, with reports of stale food, and sandwiches that weren't fresh.

Some inmates, including diabetics, said they had to go hungry if the food given to them wasn't appropriate.

Just 39 per cent of respondents were able to get into a rehabilitation programme, while 72 per cent could have a job, and half had access to education.

Those serving life sentences reported it being "almost impossible" to access any kind of programme, the survey said.

Among other concerns was their ability to receive visitors, with the majority of respondents unhappy with how many visits they were able to receive, and their length.

When they did get visitors, 64 per cent of surveyed inmates were unhappy with the chairs and space available to them, with some noting the chairs were uncomfortable for elderly visitors.

Three per cent of respondents said their prison was "exceptional", 24 per cent said it was above average, more than half (55 per cent) said it needed improvement, and 18 per cent said their prison was "sub standard".