The patient had significant mental-health issues, including anxiety and a history of self-harm and suicide.
Director of proceedings Aaron Martin said McMillan had abused his position of trust and taken advantage of the most vulnerable of patients.
"He knew the patient was at risk of self-harm and suicide. To move immediately into a sexual relationship - her first sexual relationship - was recklessly dangerous."
McMillan started texting her while she was still an inpatient. The day she was discharged, he sent her 23 text messages.
Fewer than six weeks after she was discharged from hospital, their relationship became sexual.
When McMillan visited her in Auckland, they watched the movie Sucker Punch, during which he referred to Ms NN as "Babydoll", after the movie's lead character. In the movie, Babydoll is institutionalised by an abusive stepfather in an asylum where the girls are trapped and subjected to violence and sexual exploitation. In the final scenes, Babydoll is lobotomised.
Around this time, Ms NN tried to kill herself.
McMillan lied to her twice to try to distance himself - the last time in December 2011, when he said he was being admitted to hospital. In response, Ms NN had "increased feelings of self-hatred".
Ten days later, she told a carer that her boyfriend had been her nurse.
When confronted by the Hawke's Bay District Health Board, the nurse admitted to a sexual relationship.
The tribunal cancelled McMillan's registration and ordered him to pay costs of $13,500.
It also placed strict conditions in case he ever wanted to re-register - including to be subject to two years' supervision by a health committee and to take steps to show he understood the boundaries of his profession.
