A jury has begun deliberating whether a Ruatoki grandmother is guilty of her grandson's manslaughter.

Donna Catherine Parangi has been on trial in the High Court at Hamilton for the past two weeks defending the charge after 8-month-old Isaiah Neil died after being left in a hot car for several hours in November 2015.

The Crown alleged the infant died from heatstroke, also known as hyperthermia, but the defence has suggested asphyxiation was more likely the cause of death.

On the day Isaiah died Parangi, her daughter Lacey Te Whetu and her partner Shane Neil all smoked synthetic cannabis at the Rūātoki home they shared with extended family members.

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Te Whetu and Neil have already both pleaded guilty to manslaughter.

In summing up the case for the jury of seven men and five women, Justice Sally Fitzgerald said if they agreed that Parangi had taken on a parental role on the day of Isaiah's death and that he died of heatstroke, they should find Parangi guilty.

However, if they were not convinced beyond reasonable doubt, they should find her not guilty.

Gray had put to the jury that the Crown had not been able to rule out that Isaiah was asphyxiated as he lay in bed with his parents on the afternoon he died.

Isaiah, who had been sick with bronchitis, snuffly and not sleeping well, was left asleep in his car seat while the trio slept inside after getting stoned on a "powerful batch" of the drug.

When Neil awoke he retrieved Isaiah from the car and testified that the baby was hot, limp and lifeless.

Te Whetu testified that she woke up about 3.30pm when Neil handed her the infant, and tried to give Isaiah a bottle in bed at the same time Neil returned to the bed.

When Isaiah wouldn't take the bottle she put him in his cot. At 6.30pm Te Whetu woke up again and went into a full panic when she couldn't rouse the baby.

Gray questioned what blankets were in the cot to which Te Whetu replied quilts and another big blanket.

Justice Fitzgerald said the jury would have to consider several aspects to prove her guilt including whether Parangi was acting in a role as a parent between the time he was left in the car and when he died.

The Crown said Parangi had told police in her interview that she had told Te Whetu to leave Isaiah sleeping in the car that day and therefore had taken on a parental role.

However, the defence said Parangi, 54, was the breadwinner for the whanau as the parents were not working and were home all day with their son, meaning they were acting in more parental roles.

If the jury decide Parangi did taken on a parental role they would then have to decide whether Isaiah died as a result of heatstroke or not.

The car in which Isaiah was left was photographed by police with all its windows and doors closed.

However, the defence said Parangi was insistent when she spoke to police, that she had left some of the doors and windows open.

Gray had told the jury both parents were "high as a kite" in the room with their child and asphyxiation could not be ruled out as a cause of death.

The jury would also have to take into account various evidence from experts who testified around the possible temperature of the car on the day, the impact on the baby inside and the likely cause of death.

Parangi chose not to give evidence in the trial.

The jury was sent out to begin its deliberations at 11.45am.