A Bible sits on the witness stand and was earlier placed before every juror during the trial this week of a double killer.

It's a courtroom custom used for both sworn testimony and to swear in a jury, but it's also becoming a visual reminder that it was the direct commands of God which Zarn Tarapata claims told him to "kill them".

"Them" being Paul Matthews, 47, and Paul Fanning, 69, who were brutally stabbed to death at the Takanini Ezy Cash store, where they worked, on July 19, 2014.

Tarapata, 27, is on trial in the High Court at Auckland, charged with their murders.

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He accepts he killed the two men, as they ate noodles in the lunchroom, but that it wasn't morally wrong because he was carrying out "God's work".

Tarapata's counsel, led by Jonathan Krebs, argues that his client's lack of moral recognition means he was legally insane at the time of the homicides.

Today, clinical director at the Mason Clinic for forensic psychiatry, Dr Jeremy Skipworth, continued giving evidence under cross-examination from Crown prosecutor Richard Marchant.

"The first time he heard God's voice was in the effect of 'kill them' and 'they must die together'," Skipworth said.

Tarapata had a "delusional belief" that his partner, Tamara Cassie, who worked part-time at the pawn shop, was having an affair with either Matthews or Fanning or both of them.

"He believed it to be an order from God to kill the two men he believed were committing adultery with his partner," Skipworth said.

"As I understand it in the Old Testament the punishment for adultery is death," he said.

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

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Cassie entered the store hoping to fix her phone and told Tarapata to stay in the car with her children, the court has heard.

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

He entered the lunchroom and attacked Matthews and Fanning, stabbing them to death over about three minutes.

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

Cassie has testified that she then saw a "possessed" Tarapata covered in blood and clutching a knife coming down the hallway.

She has also said she never had a sexual relationship with either of her colleagues.

The pair then returned to their car and left the scene, driving about West Auckland, with Tarapata reading his Bible - seeking direction from God, Skipworth said.

"He was confused, he probably interpreted it as a sacrifice to God which could result in God sparing his children," Skipworth, the defence's expert witness, said.

The doctor also said he did not believe all people who follow the word of God are insane.

"We can't say that all of those people are psychotic ... All consuming? Yes, that points towards it being psychotic at times," he said.

Skipworth, when referring to his clinical reports of Tarapata, said the murder-accused was struggling to comes to terms with the consequences of the killings.

"It was unimaginable to him that God's orders could be anything other that righteous," he said.

"He regretted the outcome, but struggled to understand why God had allowed him to be put in prison."

In an at-times heated line of questioning, Marchant asked the doctor: "If there was no command from God, what other explanation could there be for his actions? Anger, jealousy? Are you familiar with the term, a crime of passion?"

Skipworth replied: "If we take away the command from God? Quite possibly, it is a leading contender as to what happened."

He added that even after assessing Tarapata, a diagnosed schizophrenic, years after the homicides Tarapata "still doesn't think it's morally wrong".

"Why has God allowed this to happen, why will God no longer communicate with him?" the doctor said.

A second psychiatrist giving evidence for the defence, Dr Justin Barry-Walsh, agrees with Skipworth and said Tarapata was "floridly psychotic" at the time of the homicides.

Tarapata was also known to be making daily sacrifices to God, often by burning meat as a gift, the court has been told.

The Crown argues Tarapata knew what he was doing was morally and legally wrong.

Tarapata washed his hands, clothes, and seemingly made efforts to discard the knife after the attack before being arrested in Huntly about midnight that day.

The trial continues.