Te Whatu Ora (Counties Manukau) is conducting an investigation into an alleged assault of a patient by staff at unit of Middlemore Hospital.
It's alleged to have happened at the hospital's Tiaho Mai Mental Health Unit, by a staff member last month.
The facility provides 24-hour treatment for people with serious or complicated mental health issues.
The family of a 24-year-old allege he was assaulted on 22 October after refusing to take medication. They claim he was punched in the face and kicked by a staff member while on the ground.
The patient suffers from bipolar disorder and has been in the unit since October. Local Democracy Reporting has chosen not to identify the patient.
People who suffer from bipolar disorder often experience periods of elevated (known as mania) and low moods (depression). The condition was previously known as manic depression.
The man's brother said he was left with bruising on his legs and ribs after the incident.
"We're frustrated because we want something to happen and my brother is still in there and he doesn't feel safe," he said. "It has affected our whole family. [He's] been in the unit for almost three weeks now."
He said he has now filed a formal complaint with the Health and Disability Commissioner.
In a statement Te Whatu Ora (Counties Manukau) spokesperson confirmed it was investigating.
"Te Whatu Ora operates with zero tolerance to violence and aggression and we take this allegation seriously," he said.
"While enquiries to date have found no evidence to support the claim, the service continues to investigate and we expect this to be concluded by the end of the week."
The spokesperson said it was unaware of any such case being referred to the police, but said it would cooperate fully with any investigation.
Te Whatu Ora was asked for the title of the staff member accused of the assault and if the employee had been stood down subject to the completion of the investigation, but did not address those questions.
Local Democracy Reporting is Public Interest Journalism funded through NZ On Air