The Ombudsman has released a scathing report into Christchurch Men's Prison - one of the country's biggest.
During an inspection, 67 percent of prisoners who filled out a survey said they felt unsafe, and 49 percent said they have been assaulted.
Read the whole Ombudsman's report (1.46MB) here
The Ombudsman said conditions for at-risk prisoners were "unacceptable", and there was no suitable accommodation for people suffering from mental illness or who may be contemplating suicide.
He said high-risk prisoners were also spending too much time locked up, and lacked access to meaningful activities when unlocked.
Chief Ombudsman Peter Boshier said he was disappointed that many of the issues identified at the inspection had been raised in the past.
The unannounced inspection was undertaken under New Zealand's obligation to United Nations conventions against torture.
Several recommendations have been made to address the problems.