Prison staff and inmates are at risk because of a lack of mental health services in prisons, Chief Ombudsman Beverley Wakem says in her annual report to Parliament.
There was a "gap" in the system that defined mental health conditions, meaning more mentally ill people were in prisons than would be expected, she said.
The lack of mental health care available to these people placed other prisoners and staff at risk and may contribute to prison suicides.
Ms Wakem said during visits to prisons, her staff noticed a number of prisoners suffering from mental illness or personality disorders.
"This issue concerns us greatly. We would urge that all prisoners with mental illness who need access to inpatient beds should be able to be provided with this without delay.
"The present unsatisfactory situation places other prisoners and staff at risk. It undoubtedly plays a part in the incidence of self-harm and suicides in prison, and it makes management of the prison more stressful with staff who are not experts struggling to assess and manage these prisoners."
Senior corrections and health experts need to have more urgency in dealing with the situations, Ms Wakem said.
Her claims were supported by research by the Mason Clinic which found most major mental illnesses were overrepresented in prison.
The study estimated about 15 per cent of inmates should be receiving mental health care but, with increasing inmates and a lack of inpatient beds available, mental health services were increasingly under pressure.
The Ministry of Health and Department of Corrections were working towards improvements in prison mental health services, she said.