One in three Spring Hill's prisoners say they've been assaulted during their sentence, according to a new report.

But only one in three of those assaulted had reported it and didn't believe that any action would be taken.

Greens criminal justice spokesman David Clendon said what's even more concerning is the revelation the majority of the assaults go unreported.

"The inmates clearly have so little confidence in the system that they're not even bothering to report that. Also, there'd be an element of fear in that and the inmate reporting it would, frankly, be setting themselves up for another."

Around the same number of prisoners said there was not a member of staff they could turn to for help with a problem, and some prisoners believe there wouldn't be confidentiality if they reported an issue.

Corrections Minister Louise Upston said the inmates should have confidence in reporting incidences of violence.

Ms Upston said they've beefed up the independent inspectorate and put in place the Prisoner Welfare Governance Board.

"So when these reports are made, whether it's the Ombudsman or whether it's the inspectorate, there's the ability for Corrections to take even further steps to improve what's happening in the prisons."

The ombudsman's report was based on an unannounced inspection of Spring Hill Corrections Facility in February.

Just under half of prisoners said they felt unsafe at the time of inspection.

Mr Clendon said this is one of our newer prisons, it opened near Meremere in 2007, and yet the management is completely shot.

"If you were to design a regime for managing a prison for the purpose of increasing violence and tension, then I think it would look something like what the Ombudsman's report says is going on inside Spring Hill at the moment," he said. 

Chief Ombudsman Peter Boshier recommended more staff interactions with prisoners to improve security and prisoner staff relationships, and an anti-bullying strategy needs to be implemented, alongside the gang management and violence reduction strategy.

Mr Clendon is not happy with the findings that show youth offenders can be locked up for up to 20 hours at a time.

"Locking young people away for 20 hours a day is absolutely counter-productive towards any form of rehabilitation."

The toilets were unscreened and visible on CCTV, and those screens could be viewed by anyone entering the staff base and "presented a significant privacy issue".

Though Mr Clendon's said: "That's just intolerable. It's inhumane and it is tantamount to torture. We need to do better."

However, Corrections says its for the safe management of prisoners at risk of self-harming.

Prisoners were critical about being able to get sufficient clothing, bedding and toiletries as well as arrangements for access to their property, and mail distribution.

Judge Boshier said the lack of 'prison essentials' was of concern and there was a lack of towels, sheets, pillows, clothing and footwear.

At the time of inspection, Spring Hill Prison housed 1038 male prisoners. The prison was originally designed to house 650 sentenced prisoners at the latter stages of their sentence.

However, the function and focus of the prison has changed considerably with double-bunking and the introduction of remand prisoners, who make up 27 percent of inmates.

Judge Boshier said the prison was working hard to meet new challenges.

Human Rights Commissioner David Rutherford backed the publication of the report, saying it was the best way for detention facilities to continue to improve their practices.