A Ruatoki grandmother has been jailed for her part in the death of a baby left in a hot car while she and her daughter smoked synthetic cannabis.

Donna Parangi, 54, was today jailed for two years and six months by Justice Sally Fitzgerald in the High Court in Hamilton for the manslaughter of 8-month-old Isaiah Neil on November 2, 2015, in Ruatoki.

Parangi's lawyer, Susan Gray, pushed for a home-detention sentence but that would not have been justified, the judge ruled, given her level of culpability.

Justice Fitzgerald found Parangi's culpability was higher than that of Shane Neil, the boy's father, and similar to that of Lacey Te Whetu, who was jailed for three years for her part.

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Neil pleaded guilty to his charge and was sentenced to 10 months' home detention last year.

However, the judge accepted that despite defending the charge twice - Parangi's first conviction was quashed on appeal - she was genuinely remorseful for the death.

Gray told the court Isaiah's death had ruined her client's life and she had not touched the drug since the incident.

"This case is very sad. Sad because Isaiah lost his life and sad because he was much loved and cherished and nobody intended for this to happen," Gray said.

While Gray urged the judge to consider a total of discounts worth 50 per cent, the judge said the need to denounce this sort of offending and the fact Parangi knew she was taking on a parenting role that afternoon meant just over 30 per cent worth of discounts were warranted.

They were made up of 15 per cent for good character, 10 per cent for the hardship a prison term would inflict on her and her mental health and the remaining on compassionate grounds.

The judge said while her mental health had been impacted, that was due to her offending and were not a factor prior to Isaiah's death.

Parangi still lacked insight into her offending and felt those at fault had already been dealt with, a reason why the judge declined to issue any discount for remorse.

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Justice Fitzgerald said the crux of the case came down to culpability and the fact it was Parangi's decision to leave him in the car and she never put in alternative care arrangements for him while they smoked synthetic cannabis.

"You made the decision to leave Isaiah in the car ... when you ought to have known that Isaiah would need regular checking that afternoon. You put nothing in place to ensure that occurred, which is, in my view, the real crux of culpability in this case."

Parangi then told her daughter to go and have a sleep before she fell asleep herself.

"It was your suggestion to leave Isaiah in the car and Ms Te Whetu, as she had clearly done many times before, proceeded on the basis that you would care for Isaiah while she was under the influence of synthentics. You knew that she would not be in a fit state to care for Isaiah that afternoon."

It was Neil who ultimately retrieved Isaiah from the car, at which point he was either likely critically ill if not already dead. He was then put to bed inside.

She said there was no evidence to suggest Neil knew that Isaiah was in the car.

"At the trial before me there was no suggestions that Neil was told or aware that Isaiah was in the car. And what I accept is a perverse finding, his hopelessness as a father and that he was not aware that Isaiah was in the car that day meant his culpability was lower than Ms Te Whetu's."

She dismissed suggestions from the defence at trial that his death could have been caused by overlay - when a child is smothered by a parent while sleeping in the same bed.

She also dismissed submissions from Parangi, through her lawyer, that the car was left partly in the shade with its windows down.

The temperature in the East Coast that day was around 21 degrees. However, evidence was given at trial that a car with the windows wound up could reach 41 degrees, or as high as 46 degrees.

Justice Fitzgerald said there was no evidence to show the windows were down, including from police photos of the car taken on the day of his death.

Smoking synthetic cannabis once going inside robbed Parangi of the ability to make proper judgements of his care, she said.

"You knew that Isaiah was in the back of the car and you knew that you ought to check on him but instead you smoked cannabis knowing the effect it was likely to have on you."

However, apart from this offending, Parangi had otherwise lead a "blameless life" and often stepped in to help her daughter who was "hopelessly" addicted to synthetic cannabis.

"You are not a bad person, you are a good person who has made some very bad choices."