Bay of Plenty grandmother Donna Parangi has been found guilty of the manslaughter of her 8-month-old grandson, who died in a hot car while she smoked drugs.

A jury found the 48-year-old guilty this morning, after a six-day trial before Justice Graham Lang in the High Court at Rotorua.

Isaiah Neil (also known as Te Whetu), died after being left in a hot car while his parents and grandmother smoked synthetic cannabis and passed out at Taneatua in 2015.

Experts believed the temperature in the car would have been as high as 45C for between 90 minutes and two hours.

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The boy's parents Lacey Te Whetu and Shane Neil have already pleaded guilty to manslaughter for their role in his death.

The jury went out yesterday and continued deliberating at 9.30am today. It returned after 90 minutes.

Parangi pleaded not guilty a week ago to Isaiah's manslaughter by depriving him of the necessaries of life and failing to take reasonable steps to protect him.

Defence lawyer, Julie-Anne Kincaide, said no one disputed Isaiah's death was tragic but at the time he died he was not in Parangi's actual, legal care.

She pointed to evidence from a police interview in which Parangi said she had heard the infant crying in the house where he was with his parents.

The last time she'd seen him he'd been asleep in the family station wagon. In the interview she was adamant the car windows, doors and sun roof were open.

Kincaide disputed the crown's claim Parangi had rushed inside to consume synthetic cannabis after buying it from her dealer in Kawerau. Her account was she'd put shopping away and done two loads of washing before taking a nap.

"The defence says Isaiah's nanny was not acting in place of a parent, both his parents were there, that Isaiah was not in her actual care and there was no act or omission, no departure from the standard of care expected of a reasonable person," Kincaide submitted.

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The trial had begun at 8.15am on Monday to suit a pathologist giving evidence from London. He was the sole witness for the defence.

She will be sentenced in Tauranga, on a date yet to be set.