If lawyers for the defendant and the Crown agree, and if the presiding judge is satisfied with expert evidence that the person was legally insane, a finding of not guilty by reason of insanity can be recorded and a trial is not required.
According to the Ministry of Justice, this happens in only a handful of cases each year.
Here are some of the other Kiwi killers deemed insane by the courts and found not guilty of murder.
Akshay Anand Chand
Akshay Anand Chand in the dock at the North Shore District Court on his first appearance charged with the murder of Christie Marceau in Hillcrest. Photo / Paul Estcourt
He was on bail at the time and facing charges of kidnapping and assaulting Christie several months before she died.
Although he pleaded guilty to the earlier offending, two leading psychiatric experts deemed Chand to be insane at the time of Christie's death.
Justice Helen Winkelmann ruled that although Chand knew he had killed Christie - he admitted it, and said he had planned the gruesome attack - his mental disease meant he did not know what he was doing was morally wrong.
After Christie died, Chand was diagnosed with schizophrenia. He said a voice told him to kill Christie and he felt compelled to do it.
Justice Winkelmann ordered Chand be detained as a special patient at the Mason Clinic - meaning he will stay at the forensic mental health facility until the Ministry of Health's Director of Mental Health deems it safe to release him.
Martin Robert Lyall
Martin Lyall, who was charged w the murder of Kevan Newman in Henderson in 1995 but found unfit to plead. Photo / supplied
In 2005, Martin Robert Lyall fatally stabbed 65-year-old Kevan Newman during a lunchtime rampage in West Auckland.
Lyall, then 31, was reported to be wandering around central Henderson drinking, possibly from a bottle of spirits, before he stabbed Robert Norcross in his fishing shop.
He then went outside and stabbed Newman, who died in an ambulance on the way to hospital.
Norcross was seriously hurt, with stab wounds to his abdomen, hand and shoulder.
Lyall was shot three times by police when he refused an order to drop his knife.
As he continued to advance on police he was tackled by a council parking warden and subdued.
Lyall, who had been a mental health patient for 13 years before the stabbing frenzy, was charged with murder but later deemed mentally unfit to stand trial.
Colin Moyle was murdered by Matthew John Ahlquist in 2007. Photo / supplied
In April 2007 Matthew John Ahlquist threw boiling water on 55-year-old Colin Edward Moyle, bludgeoned him with a spade and set him on fire at his Auckland home because he thought he was "demonic".
Ahlquist, then 33, was living rough after leaving an Auckland City Hospital psychiatric unit in February.
He had been there voluntarily but was asked to leave after breaching an alcohol ban.
After meeting Ahlquist at the Sandringham shops, Moyle offered the homeless man a place to stay at his Housing NZ home in Kiwitea St.
Not long after, Moyle served Ahlquist with a trespass notice following an argument about rent. Two days later Moyle was killed.
Ahlquist was charged with murder but two expert psychiatrists later diagnosed him with paranoid schizophrenia and Justice Geoffrey Venning ruled he was not guilty by reason of insanity.
Henry Pan, also 24, was killed in his Half Moon Bay home on February 15, 2013.
Armstrong drove to Pan's home armed with a large kitchen knife, smashed through the front door and after a brief "wrestling match" with the victim stabbed him repeatedly in the neck.
One of Armstrong's thrusts was so forceful the knife went completely through Pan's neck.
Armstrong fled the bloody scene and drove home where he tried to kill himself by slashing his own wrists and stabbing himself in the stomach.
Armstrong was charged with murder but later cleared of criminal culpability after the High Court at Auckland heard from two forensic psychiatrists who agreed he was in the grip of schizophrenia when he killed Pan.
Dr James Cavney said Armstrong probably knew he was killing something but was convinced it was a demon.
Justice Graham Lang found Armstrong not guilty by reason of insanity and ordered he be detained indefinitely as a special patient at the Mason Clinic.
Stephen Lawrence Anderson
Stephen Anderson is driven from the Hamilton District Court in a police van after appearing charged with murdering six people at Raurimu in 1997. Photo / Martin Sykes
In 1997 Stephen Anderson went on a drug-fuelled rampage at a family lodge at Raurimu, killing six people.
Anderson was 25 at the time and had been diagnosed with schizophrenia when he committed one of the worst mass killings in New Zealand history, running amok with his father's shotgun at a family gathering.
His victims were his father Neville Anderson, 60, Andrea Joy Brander, 52, Anthony Gordon McCarty, 63, John Frederick Matthews, 28, Stephen Mark Hanson, 38, and Henk van de Wetering, 51.
Clark, 36, had been living with Tampin for just one week but had asked his new flatmate to move out due to "odd behaviour". Not long after that Tampin stabbed Clark 15 times with a butcher's knife.
Once the brutal attack was over, Tampin cleaned the weapon, had a shower and went to bed. The High Court heard that Tampin was a paranoid schizophrenic and knew he was stabbing someone and who it was but, according to two psychiatirsts who gave evidence, at the time he was hearing overwhelming voices telling him to kill his friend.
2013: Stephen Mark Whittaker was found not guilty of the murder of his partner Bronwyn Sadler in Christchurch by reason of insanity.
On September 27, 2011 Whittaker, 55, said he "just went crazy" and stabbed Sadler 26 times with a large hunting knife at the home they shared in Spreydon. He then phoned police and said the "crazy bitch had a knife on her" and had tried to kill him.
Whittaker, who has since changed his name by deed poll to Stephen Scipio, was found mentally impaired and not fit to stand trial, and the following month he was detained as a special patient at the forensic unit at Hillmorton Hospital.
2001: Grant William Milliken, 31, was found not guilty by reason of insanity in the High Court at Rotorua after the death of his landlady Joanna Mary Elizabeth Monk.
Milliken, a sickness beneficiary who suffered from schizophrenia, rented a unit owned by Monk and her husband Phillip. After an argument over parking Milliken attacked the couple with a knife, killing the woman and wounding her husband. After a jury trial Milliken was found not guilty by reason of insanity.
