Killer Simon Frank has joined the ranks of those acquitted of murder by reason of insanity, a group of offenders that are steadily on the rise.

Frank was charged with murdering his wife earlier this year and found to be insane at a hearing in the Rotorua High Court in October.

At a sentencing hearing this week Frank was made a Special Patient under Mental Health Legislation and ordered to be detained in a Forensic Mental Health facility to treat his illness.

Frank, 56, from Wellington, killed his 58-year-old wife while the pair were on holiday in Taupo at Easter this year.

A record number of offenders were found not guilty by reason of insanity in New Zealand in 2016 with 34 successful acquittals for various offending compared with only four acquittals in 2000.

Victim advocate Graeme Moyle has long campaigned for a change in the verdict citing it's unfairness in terms of delivering justice to victims.

"To state, in Court, that an offender who is known to have committed an unlawful act, is found to be not guilty of that act is abhorrent to any survivor or victim and denies them justice" says Graeme.

"The verdict is wrong, archaic and after 180 years in existence is well overdue for reform".

"It may well suit lawyers, judges and psychiatrists" says Graeme, "but it is offensive to those who have suffered at the hands of an insane offender".

"I have championed a verdict of Proven But Insane for many years and believe this simple change will bring a great deal of relief to victims knowing that the Courts actually acknowledge the defendants offending".

"It is time we put the victims first in these cases and not victimise them further with the current ‘not guilty' verdict".

"Insanity acquittals have continued to rise steadily over the last 17 years and with Police attending over 40,000 incidents a year involving the mentally ill it stands to reason that these types of acquittals will continue to increase."

"Increase in acquittals equates to an increase in victims denied their right to justice. Reform is the only option to balance the books and give the victims the accountability they deserve".