New Zealand's mental health system is failing and regressing to the old "out of sight, out of mind" days, according to the Green Party's health spokesperson Kevin Hague, who is calling for an urgent inquiry into the system.

The Chief Ombudsman has labelled the living situation of autistic man Ashley Peacock as cruel, inhuman or degrading, prompting fresh calls for him to be removed from near-permanent seclusion.

Ashley has been kept in a tiny wing of a mental health unit at Porirua for five years, allowed outside for an average 90 minutes a day.

He sleeps in a 10m-square room with just a mattress and a urine bottle, and when staff order it, can be locked in for long periods - despite repeated warnings from multiple agencies that his condition is deteriorating, and his treatment breaches human rights.

Kevin Hague said the case was an example of why an inquiry was needed.

"It reminds me of the bad old days where people were classified as being too hard and they were left in the back ward of the big old psychiatric hospitals."

Hague accused Health Minister Jonathan Coleman of sitting back and watching mentally ill New Zealanders suffer.

"If that is acceptable to Jonathan Coleman then I think the New Zealand will have something to say about that."

If it is not acceptable, the minister needed to act, Hague said.