A young woman who admitted culpability in the death of her baby will now give evidence in her mother's trial on the same manslaughter charge.
Isaiah Neil was eight months old when he died at his grandparents' home in Rūātoki, near Whakatāne, in November 2015.
The infant died from heatstroke after being left in a hot car for several hours, according to the Crown which alleges his grandmother, Donna Catherine Parangi, is partly responsible for his death.
Parangi pleaded not guilty to the manslaughter charge.
Her legal team, Susan Gray and Julie-Anne Kincade, said there was "considerable doubt" that heatstroke, also known as hyperthermia, was the cause of death.
"The evidence is equivocal, if not weak. Science is not on the side of the Crown in this case," Gray told the jury.
Asphysxia, or smothering, could be another possible cause of death, said Gray.
Two of the key witnesses for the Crown are Isaiah's parents Shane Neil and Lacey Te Whetu, who is Parangi's daughter.
The pair previously pleaded guilty to the manslaughter of their son, the jury in the High Court at Hamilton was told.
In opening the Crown case, prosecutor Richard Marchant said the trio all lived together at Parangi's home in Rūātoki.
Parangi and Te Whetu were the primary caregivers for Isaiah, said Marchant, while Shane Neil did "virtually nothing".
As such, Marchant said Parangi had a duty under the law to take reasonable steps to protect Isaiah.
"Quite simply, because of synthetic cannabis, she did not," said Marchant. "Baby Isaiah was left in a hot car as a result."
The infant was left in the car, in direct sunlight on a warm spring day, for around three hours according to the Crown case.
Parangi and Te Whetu were both addicted to synthetic cannabis and were smoking the drugs inside the house while Isaiah was in the car, said Marchant.
Shane Neil also smoked synthetic cannabis that day and the trio all fell asleep.
While Parangi was described as a good grandmother who helped her daughter take care of Isaiah and her other grandchildren, Marchant said witnesses would say these potent drugs turned Parangi into a "zombie" sometimes unable to move.
"Isaiah, because of his age, was unable to communicate, unable to get himself out of the car seat, unable to open windows," said Marchant. "As a result, the Crown says, he died."
Baby Isaiah was described by witnesses as "hot", "limp" and "lifeless" when he was removed from the car, said Marchant.
Attempts to resuscitate Isaiah, by Lacey Te Whetu at first and then ambulance paramedics, were in vain.
He was pronounced dead at 7.21pm on November 2, 2015.
Donna Parangi, Lacey Te Whetu and Shane Neil were jointly charged by police with manslaughter after a six-month investigation.
Te Whetu and Neil pleaded guilty to the manslaughter of their son, said Marchant, and will give evidence at Parangi's trial in the High Court at Hamilton.
"That's going to be a very difficult exercise," said Marchant.
A weather expert would give evidence to say the minimum temperature in the region was 20C, said Marchant, while a heat expert would say the temperature in the car could exceed 40C.
In the opinion of two pathologists, Marchant said Isaiah died from injuries consistent with hyperthermia – more commonly known as heatstroke.
The pathologists said their findings on the cause of death were "inconclusive" and also noted the presence of extensive bruising on Isaiah's body, as well as an adult bite mark.
While these injuries were concerning, Marchant said they were not considered to be the cause of death.
In her opening address, Parangi's lawyer Susan Gray said the death of Isaiah was an "absolute tragedy" but the case was complex, both legally and factually.
And while smoking synthetic cannabis was addictive and destructive, she said Parangi was not on trial for smoking the drug.
"Ms Parangi is on trial for manslaughter as the Crown says from hyperthermia from a closed, sealed car," said Gray.
"The defence agree Isaiah was left in the car. The defence disagree [the car] was closed."
As well as raising the possibility of asphyxia as the cause of death, Gray disputed the Crown assertion that Parangi was acting in the role of a parent on the day Isaiah died.
Her client was on the first day of a well-earned holiday from her job, said Gray, while Isaiah's parents were unemployed, full-time parents.
"This is a very serious matter for Ms Parangi. We ask you to keep an open mind."
Shane Neil was the first witness in the trial. He described a household where smoking synthetic cannabis was commonplace.
On the day Isaiah died, Neil said he got "stoned" on the drugs after Lacey Te Whetu and Donna Parangi had been smoking. All three fell asleep.
When he woke up, Shane Neil said he went looking for Isaiah. He couldn't find him inside the house, so went outside to the car.
Neil found his son strapped inside his car seat, unresponsive.
"[He was] sweaty, wet, hot," said Neil. "Limp, he felt limp."
Neil took Isaiah inside and tried to check for a pulse.
"I couldn't tell, I was still pretty high. He did appear to be lifeless at the time, that's the only way I can describe it."
Neil was to continue to give evidence on Tuesday.
The trial in front of Justice Sally Fitzgerald in the High Court at Hamilton is expected to last up to three weeks.