A Bay of Plenty grandmother left an 8-month-old boy in a hot station wagon with the windows up while she and the baby's mother binged on synthetic cannabis, the Crown alleges.

The accusation was levelled against Donna Catherine Parangi, 48, of Ruatoki, by Crown prosecutor Anna Pollett in the High Court at Rotorua today.

Parangi has pleaded not guilty to the manslaughter of Isaiah Neil in November 2015, by depriving him of the necessities of life and failing to take reasonable steps to protect him.

The baby's parents Lucy Te Whetu and Shane Christopher Neil earlier pleaded guilty to a similar charge.

Opening the Crown's case Ms Pollett claimed through their drug use Parangi, her daughter and Neil played Russian roulette with children in their care, Isaiah included.

She outlined how, around 9.30am on November 2, Parangi and Te Whetu took the baby with them to buy $40 worth of synthetic cannabis.

It was a hot day, and once home they left Isaiah asleep in the car with the windows up while the trio smoked until they fell asleep. At no stage did they check on the baby.

Ms Pollett said it wasn't until other children returned from kohanga reo and Neil woke up that he took Isaiah out of the car, where he'd been at least three hours.

Despite finding him hot, sweaty and unresponsive he'd done nothing for the infant or called emergency services. Instead, he'd put him beside his sleeping partner where he'd remained for another three hours.

When an ambulance was finally called it was discovered Isaiah was dead from heatstroke.

Ms Pollett said evidence would be called that the child would have died within 45 minutes inside the car.

She contended Parangi had actual care of, and responsibility for, her grandson saying both she and her daughter consumed the synthetic cannabis "without concern for baby Isaiah".

"The fatal decision she made along with her daughter was the consumption of synthetic cannabis over the care of her mokopuna."

Ms Pollett said a large bruise and other non-accidental injuries were later found on the infant's body.

Defence lawyer Susan Gray disputed the crown's claim Parangi departed from the standard of care expected, saying the main issue would be whether she was in the state of a parent at the time Isaiah died.

She asked the jury to note the parents' guilty pleas, saying these were entered because they were the ones who had care of him but failed him.

She also took issue with the crown's contention that Isaiah had died from heatstroke.

"The defence says this is far from clear."

The trial is expected to last two weeks.

-NZ Herald