The public watchdog found the patient being kept in almost constant "seclusion" - solitary confinement in a bare room - at the mental health unit of a district health board.
Chief Ombudsman Beverley Wakem would not name the board last night, but said it claimed the detention and use of restraints was because of the patient's propensity to attack other patients and staff.
But Ms Wakem said the board moved the patient to a more suitable facility when it was confronted by her office.
"Why nobody thought to look at that and make that assessment before we arrived on the scene is a cause for concern," she said.
The patient was one example of "potential cruel and inhumane treatment" the Ombudsman identified during the nationwide investigation.
It found a young intellectually disabled patient being kept in unwarranted and lengthy "seclusion", and another mental health patient who had been kept without any consent for some years.
Ms Wakem said the health boards responsible took action immediately.
However, Health Ministry director of mental health Dr David Chaplow knew nothing of the cases last night and said he would be ordering an urgent report.
He said he was aware of one patient with a mixture of autism, intellectual disability and mental illness who was particularly challenging "but I have never known a case in seclusion for six years".
There were 1395 patients secluded for between two minutes and 365 days in the past year, according to the annual mental health services report.
Dr Chaplow said there was now a "sinking lid" policy on seclusion, which had halved its use.
But seclusion had a place in mental health care, and he disagreed with the "ideologues" who said it should be banned altogether, because the alternative was "people locked up in police cells, people in handcuffs, or tied to a tree".
The Ombudsman's investigation covered prisons, mental health units, immigration detention centres, court cells and youth facilities.
It was triggered by the Ombudsman's new legal designation as the "national preventive mechanism" under New Zealand's commitment to the United Nations Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment.
The investigation, detailed in the Ombudsman's annual report which was released yesterday, also raised concerns that prisoners were not being provided with electric fans to control the ventilation or temperature of their cells.
It said in excessive temperatures the lack of fans could amount to "cruel" or "inhumane" treatment under the legal definition of torture.
Corrections prison services manager Karen Urwin said the department had looked into buying fans for every prison cell but decided it was not an effective use of taxpayer money.
Prisoners could buy their own fans through the process for acquiring televisions and radios.
The Ombudsman's other concerns included patients with dementia or Alzheimer's disease being held when "informed consent" had lapsed.
The cases
- Mental health patient in "virtually constant restraint and seclusion for nearly six years".
- Young intellectually disabled patient kept in "seclusion" for lengthy period.
- Mental health patient "treated for some years without any apparent consent of any kind".
