Staff at a Southern District Health Board unit stripped a young woman naked and left her with no mattress or pillow for at least 12 hours overnight on a "cold hard surface" in seclusion in a mental health unit.

The report by the Mental Health Commissioner Kevin Allan found the Southern DHB's actions breached seclusion guidelines, were unacceptable and unkind, and failed to respect the woman's dignity and independence.

However, in investigating the 2013 incident, Mr Allan said he could not find that denying her clothing and bedding was intended to be punitive or to humiliate her.

The young woman - who at the time was in her late teens - had significant depression and had self-harmed, and had been in a psychiatric unit at a public hospital under a compulsory in-patient treatment order under the Mental Health Act.

She left the clinic one day and was found by police officers, who took her to a secure unit. She was secluded at about 8:30pm after being seen by a doctor. There, her clothing was removed, but she was not given a tear-resistant gown to wear, nor provided with a mattress or pillow. She was left with only a tear-resistant blanket and a cardboard bedpan, and the lights in the room were left on all night.

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"Ms 'A' said that it was extremely uncomfortable trying to sleep on the cold hard surface, especially as the blanket would not wrap around her completely, and she did not want to lie on the top of the blanket because the nurses observing her through the window and door would see her naked. She stated that the staff left the lights on full all night, so it was impossible to sleep," the report said.

Southern DHB said: "We are sorry there was a delay in offering the gown to [Ms A]. It is unclear from the clinical records why this did not occur sooner. This is not standard practice, although such interventions need to be individualised to ensure the safety of the patient involved."

Nurses undertook 10-minute observations, and two-hourly assessments, and at 4:30am two nurses recorded an eight-hourly assessment. At 8am staff went into the room to give her food and a drink.

She was only provided with a gown and mattress at 11am. The seclusion was suspended about 1pm, after about 18 hours, and the woman was returned to the clinic she was initially at.

The report found a number of Southern District Health Board staff failed to comply with the health board's own seclusion guideline, the seclusion requirements of the Ministry of Health, and with the accepted standard of care for nursing staff.

"My expert advisor, Dr O'Brien, advised that it was not reasonable that Ms A was not provided with a gown, mattress, or pillow," Mr Allan said.

"He stated: 'Apart from having to endure the discomfort of sleeping on the floor with no support for her head, it is undignified for anyone to be deprived of all clothing.'

"Dr O'Brien noted that Ms A was being observed every 10 minutes, and that this could have been increased to continuous observation if it was thought that there was a risk that she would use a gown to harm herself. He said that it is hard to imagine how a mattress and pillow could be considered a risk."

Southern DHB said: "The clinical notes indicate that Ms A displayed a high level of volatility and a high self-harm risk, and that she was unwilling to provide assurance or willingness to be safe or to co-operate with the healthcare providers present."

The health board agreed to apologise in writing, and undertake training on the relevant Code of Rights, and review other policies.

"Southern DHB said that its current seclusion guidelines are less restrictive, and allow patients to retain normal clothing, the same bedding as the rest of the unit, and access to an en suite bathroom and water," the report said.

It also said the health board has trained staff in seclusion reduction strategies, and is working towards a zero seclusion goal by 2020.

All names in the report were anonymised, apart from the expert consulted for the report, Dr O'Brien, and the DHB.

The woman said it was probably the most humiliating and dehumanising thing she's ever experienced, and she's terrified of getting unwell again.

"As set out above, a number of Southern DHB staff failed to comply with the Southern DHB seclusion guideline and the Ministry of Health publication, and with the accepted standard of care for nursing staff... Southern DHB failed to ensure that staff complied with its policies and provided care of an acceptable standard in a number of ways," Mr Allan said.

"In my view, the manner of seclusion, over a period of approximately 18 hours, including removing Ms A's clothes, not providing her with a mattress, pillow or gown, and not dimming the lights overnight, meant that Southern DHB failed to respect Ms A's dignity and independence and, accordingly, breached Right 3 of the Code of Health and Disability Services Consumers' Rights."

Southern DHB agreed to apologise to the woman, and undertake further staff training on restraint, seclusion and the Code of Rights, and review its restraint minimisation and seclusion guidelines. It also agreed to review the seclusion policy to specifically state what clothing and bedding should be provided when people are placed in seclusion.