One in three inmates surveyed at Spring Hill Corrections Facility in Waikato claimed they were assaulted inside - but most assaults were not reported to Corrections.
Many inmates did not tell guards because they had no confidence in the staff, according to findings in Chief Ombudsman Peter Boshier's new inspection report.
It follows a similar report last month that highlighted concerns about prisoner safety and bullying at Hawke's Bay Regional Prison.
In the survey of about 560 prisoners at the Waikato prison, more than half of the Spring Hill respondents reported feeling unsafe at some time.
Nearly as many said they felt unsafe at the time of the inspection.
The report said recent increases in prisoner numbers due to the introduction of remand prisoners left staff stretched, placing systems and governance under pressure.
Spring Hill can house 1038 inmates.