The public's health watchdog says an unnamed male nurse sexually exploited an acutely unwell, young mental health patient.
Health and Disability Commissioner Anthony Hill released a report on the case on Tuesday.
Mr Hill says the nurse in his mid-30s befriended the then 19-year-old when she was a mental health inpatient at an unnamed public hospital in April 2011.
He says she was vulnerable with a history of depression and anxiety, and episodes of self-harm, but the nurse befriended and then began contacting her following her discharge from hospital.
A sexual relationship began within six weeks of her leaving hospital, which the commissioner said is against professional and ethical standards.
He did not accept the nurse's submission that his own health issues meant he shouldn't be held responsible.
The commissioner referred the man, who is no longer working as a nurse, for possible follow-up legal action.