Clayton Weatherston did not have a disease of the mind when he stabbed and cut his former girlfriend 216 times, but is a "vulnerable character" prone to narcissistic rage, a psychiatrist says.

Weatherston had the features of an anxiety disorder, was obsessive and may have been affected by an increased dose of anti-depressants and lack of sleep when he killed Sophie Elliott, Dr David Chaplow told the High Court at Christchurch.

"I am of the opinion that he suffered no 'disease of the mind' at the time of the killing."

Yet there was little doubt that Weatherston "dissociated" during the frenzied attack on Miss Elliott.

"The person experiencing dissociation often states they have no 'feeling' during events and that they have patchy memory for events."

Dr Chaplow, New Zealand's director of mental health, said he interviewed Weatherston in prison twice after the killing.

Weatherston was controlling and "appeared to have an inflated sense of his own achievements", but told Dr Chaplow he felt nauseous after reading details of what he had done to Miss Elliott.

"When I asked him whether he regretted his actions, he tearfully replied: 'There are many things I regret."'

Weatherston, a former Otago University economics tutor, is on trial for the murder of Miss Elliott, but says he is guilty only of manslaughter because he was provoked.

"Weatherston was and is a vulnerable character because of his personality characteristics of anxiety, obsessionality and narcissism, and made more so due to his two nights of difficulty sleeping [before the killing]," Dr Chaplow said.

"I am mindful that one of the character traits of the narcissist is the propensity for 'narcissistic rage' when frustrated and/or humiliated."

Miss Elliott allegedly made a comment about Weatherston's mother just before the killing. Dr Chaplow said Weatherston had a dependence on his mother, and wet his bed at a later age than normal.

"He was tearful when occasion meant that he was to leave home."

Weatherston had increased his intake of the drug Prozac by three times in the days before the killing.

"It may not be unreasonable to believe that the excess medication played a part."

Dr Chaplow said Weatherston was at a loss to explain his mutilation of Miss Elliott.