He said the patient, known as Ms N throughout the hearing, was "very vulnerable" at the time of the charges.
"It was simply not appropriate for Mr Shennan to enter into the relationship that he did with Ms N. He was aware what he was doing was quite wrong, and serious.
"He must also have been aware of Ms N's vulnerability and that entering into such a relationship would inevitably have increased the risk to Ms N. This is quite contrary to a nurse's role to provide the patient with safe and effective care thereby minimising the risk of any harm."
He said privacy of patient information in mental health "should not under any circumstance be abused."
Mr McClelland added: "Accessing Ms N's private health information when he had no authority to do so amounts to a serious abuse.
"The admitted conduct in respect of both charges falls seriously short of accepted standards, amounts to malpractice and/or is conduct which has or is likely to bring discredit to the nursing profession and that a disciplinary sanction is justified for the purposes of protecting the public, maintaining professional standards and punishing Mr Shennan."
A statement of facts agreed by both parties stated the woman was known to the Mental Health Services in Christchurch with a diagnosis of borderline personality disorder.
It said she had "complex social issues, mood problems, suffered from depression and personality issues".
She told the nurse's colleagues she "felt uncomfortable about the relationship, thinking it to be unprofessional".
The woman added that "by texting and going to the movies professional boundaries had been broken".
While the patient did did not believe there were sexual overtones in Shennan's conduct, the PCC argued there was "very much potential for an intimate or sexual relationship to develop".
She also raised fears with other nurses that she did not want to complain about his conduct because she feared she would be blamed for the situation, or that no one would believe her or "take her seriously."
Shennan, of Rolleston, outside Christchurch, was sacked from the Christchurch District Health Board on February 5 last year.
He told the tribunal today he acknowledged what he did was "wrong" but pleaded not to be struck off the register.
He said he had undertaken counselling and regular supervision sessions.
Shennan, a nurse of 30 years, added: "I have worked through my issues of what proper boundaries are."
Asked by Mr McClelland if he accepted that from the outside it could appear as if he was "grooming" the patient, he replied: "I can see how people might get that impression, but that certainly wasn't the case."
The PCC tribunal ruled that Shennan - who has a new nursing job in Christchurch which was suppressed by the SCC - be subject to a supervision order for at least 18 months.
The tribunal also made an order of censure, with chairman Bruce Corkill highlighting their "strong disapproval" of Shennan's conduct.
Shennan was also ordered to pay costs amounting to $5000.
