Five homicides within 15 months involving acutely unwell mental health patients have led to an overhaul of Wellington health systems following a critical review.

The review highlighted chilling missteps, including what it terms a "tragic failure of technology", when a GP's alert to a patient's mental state slipped through the cracks.

The GP sent a fax to crisis assessment teams (CAT) recording how "acutely unwell" a patient was, but the fax was not received. The fax was sent on the same day as one of the killings.

It also highlighted problems around decision-making over when to admit acutely unwell patients into compulsory care.

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It said treatment was given on a "least restrictive" basis which led to one patient being given outpatient care at home, in order to preserve their autonomy. But it found clinicians may have given "insufficient importance" to the patient's need for more intensive inpatient treatment.

The review was ordered last year after a spate of killings by different mental health patients over 15 month in 2015 and 2016, including the killing of Kapiti nurse Cathy Stewart.

The 56-year-old Kapiti nurse was killed on February 14, 2015 in her home. The 30-year-old man responsible for her death was under the care of Capital and Coast DHB at the time, but on leave from his temporary community-based accommodation.

He was acquitted of murder on the grounds of insanity, with his name and details permanently suppressed.

He has now been made a special patient, which means he is held in a forensic unit at a psychiatric hospital.

The review covered Capital and Coast DHB, Hutt Valley DHB, and Wairarapa DHB.

At the time of the review, two of the mental health patients had been found not guilty of homicide on the grounds of insanity, one found guilty of murder, and two were yet to come to trial.

The victims' families were invited to take part in the review, and two families accepted that invitation.

Eight recommendations have been made, including around record keeping and training systems.

The DHBs' general manager for mental health and addictions, intellectual disability services, Nigel Fairley, said the incidents that led to the review were upsetting for the families, clients, and staff.

Some of the necessary changes were already being worked on at the national level, including a move to a single electronic health record.

The current system involved both paper and electronic records from a variety of different clinicians, which meant crucial information could be missed.

"Part of the issue is that people move around, and that's not unusual for clients of our service, for them to move around between DHBs," Fairley said.

"We have also developed a whole new policy around family involvement and the way in which clinicians need to more significantly involve families in any assessments."