A double-killer who says he was commanded by God to brutally slay two men was tried for the same murders twice, before eventually being found not guilty by reason of insanity, it can now be revealed.

After a final disposition hearing today, the Herald can now publish details of Zarn Tarapata's two-week retrial last October and November in the High Court at Auckland, after which a second jury acquitted him of the murders of Paul Matthews, 47, and Paul Fanning, 69.

The first jury had found Tarapata guilty in May 2015.

Matthews and Fanning were stabbed to death at the Takanini Ezy Cash store, where they worked, on July 19, 2014.

Fanning was stabbed six times in his chest and neck, and Matthews was stabbed 15 times. His throat was also cut.

After the initial guilty verdict, Tarapata, a 27-year-old diagnosed schizophrenic, was sentenced to life imprisonment with a minimum non-parole period of 17 years on July 9, 2015, by Justice Pamela Andrews.

The first trial

At the first trial, Tarapata pleaded insanity and claimed God had instructed to carry out the killings.

The Crown prosecutors and defence team agreed he was psychotic at the time of the stabbings, but they were divided over the cause, whether Tarapata knew the consequences of his actions, and if he had a "disease of the mind".

The defence claimed it was a disease of the mind and schizophrenia that had deprived Tarapata of moral understanding.

The Crown blamed methamphetamine use.

Under New Zealand law a defendant accused of a crime is presumed sane and it is up to the defence to prove insanity.

Tarapata's first trial lawyer, Bruce Hesketh, argued his client was psychotic and that mental health should be at the forefront of the court's considerations.

However, the jury went with the Crown's version.

When sentencing Tarapata, Justice Andrews reduced his sentence for mental-health issues and "sincere remorse".

The appeal

A year later Tarapata appealed the jury's verdict and conviction at the Court of Appeal and adduced fresh evidence that when he was drug-free in prison he had been diagnosed with treatment-resistant schizophrenia.

He also challenged the handling - by his own defence counsel, the Crown and its expert witness - of factual evidence about his drug consumption in the months immediately before the killings.

He further challenged the Crown's use of propensity evidence without direction to the jury.

At the Court of Appeal, Justice Forrie Miller, Justice Patricia Courtney and Justice Peter Woodhouse said they were "satisfied that this is a clear case of counsel error in the handling of both the evidence of drug use in fact and the propensity evidence".

They said the error "has occasioned a miscarriage of justice".

Tarapata's convictions were set aside and a retrial ordered.

The Court of Appeal also suppressed any publication of its decision until final disposition of the retrial to protect Tarapata's right to a fair second trial.

The Crown went forward with a second prosecution and trial and sought again to convict Tarapata.

The retrial

At the retrial, defence lawyer Jonathan Krebs, who helped free Teina Pora, argued on behalf of Tarapata.

And this time the Crown was prevented from introducing any propensity evidence about Tarapata's drug use.

However, before the trial began, Krebs said he was still concerned about a potential influence on the jury given the extensive online news coverage of the first trial, much of which referenced Tarapata's meth use.

The High Court briefly served New Zealand media organisations, including the Herald, with a "takedown" order, which mean all articles about the case had to be removed from news websites before Justice Simon Moore reversed his decision a few days later.

When the retrial began, like the first, Krebs argued Tarapata was legally insane and believed he was carrying out "God's work".

The court heard that Tarapata made daily sacrifices to a higher being by burning meat in his backyard, and claimed God spoke to him and instructed him to kill Matthews and Fanning for their sins, which to him was an affair with his partner Tamara Cassie.

Cassie, who also worked at the store, never had a sexual relationship with either man.

Krebs argued that because his client believed God was the highest moral authority, he lacked any awareness for the wrongfulness of his actions.

Crown prosecutor Richard Marchant instead argued Tarapata was aware of his actions and had killed Matthews and Fanning after being overcome by jealousy.

Moments before the killings, the court heard, that Tarapata arrived at the Great South Rd shop with Cassie and their children.

Cassie entered the store to have her cellphone repaired and told Tarapata to stay in the car with her children.

But Tarapata left the car and snuck around to the rear of the business.

He entered the lunchroom and butchered the men over three horrific minutes.

A "wild and possessed" Tarapata covered in blood and clutching a knife was then seen coming down the hallway, Cassie testified.

The pair then returned to their car and left the scene as Tarapata, reading his Bible, told Cassie to drive around West Auckland.

They stopped near the Avondale Racecourse and Tarapata washed his hands, clothes, and seemingly made efforts to discard the knife.

He was arrested in Huntly after walking into the police station at midnight on the day of the attacks.

Dr Jeremy Skipworth, the defence's expert witness and the clinical director at the Mason Clinic for forensic psychiatry, said in his opinion Tarapata was insane.

A second psychiatrist, Dr Justin Barry-Walsh, agreed, saying Tarapata was "floridly psychotic" at the time of the homicides.

Arguing against the defence was psychiatrist Dr David Street who told the jury he believed Tarapata's thoughts were fluid during the killings.

This time, however, the forewoman rose and told Justice Moore "not guilty".

A history of mental illness

Tarapata was not being treated by mental health services at the time of the killings, court documents obtained by the Herald show.

But his family has a history of schizophrenia and a history of unusual behaviour consistent with mental illness, including religious delusions and persecutory beliefs about sexual fidelity and family safety.

"His religious behaviour manifested itself in 'sacrifices' in which he burned meat, and a habit of opening the Bible to find direct instructions from God. It is said that when he killed the victims he was acting on an instruction given to him by the voice of God," the Court of Appeal ruled.

Tarapata's persecutory beliefs also extended to accusing his own father of having an affair with Cassie, the judges said.

He would also known to arm himself with knives and stay up all night to protect his children.

Today, Justice Moore, quoting medical reports, said Tarapata continued to pose a high risk to others and may relapse into the state he was in when he killed Matthews and Fanning.

He has also fashioned stabbing weapons while in custody and has delusions about harming others, Justice Moore said.

The judge said if Tarapata were to be released it would expose the public to an unnecessary risk of violence.

Tarapata was remanded as a special patient to a hospital by Justice Moore.

Given the lack of serious violence in his past, and little evidence he had a temper or jealousy issues, there was no other way to explain the defendant's actions, lawyer Jonathan Krebs argued.

"(Insanity) is not a defence in a sense that is some sort of legal construct that permits the guilty to go free," he quoted.

"It is a recognition by the law that a person may ... be so disordered in their thinking, that at the time they lack the capacity to be held responsible."

Tarapata will now be held at Auckland's Mason Clinic indefinitely until discharged by the Minister of Health.