A car that baby Isaiah Neil was left in for several hours would have more than doubled in temperature within 50 minutes, a court has heard.

Scientist and temperature expert Dr David White told the High Court at Hamilton the temperature inside a car can increase by 20C in direct sunlight.

The outside "ambient" temperature at Tawera, near Rūātoki, on November 2, 2015 was estimated by White based on weather reports to be 21C, meaning the inside of the car would have climbed to a minimum 41C if all the doors and windows were shut.

Isaiah's grandmother Donna Catherine Parangi is on trial for the manslaughter of her infant grandson.

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The 8-month-old baby's parents Lacey Te Whetu and Shane Neil have previously pleaded guilty to the same charge.

On the day he died Isaiah was left by Te Whetu and her mother Parangi in his car seat in the station wagon parked outside the home the baby shared with his parents, grandparents and two older siblings.

Te Whetu and Parangi had been on a drug run to Kawerau where they bought synthetic cannabis from a cousin, arriving home about 12.30pm.

They went inside and together with Neil smoked the synthetic cannabis before falling asleep.

Baby Isaiah had been sick with a cold and was left asleep in the car, wearing a long sleeve shirt and shorts.

White took into account scene reports, meteorologists reports, aerial photographs, a diagram of the position of the house and car, wind factor and sea breezes, the colour of the car, tints on the windows, and the position of Isaiah's car seat before forming his opinion.

Based on research he said it was more likely the inside temperature that day would have been 45C or more.

He said tinted windows, which the two-tone Navy blue 1997 Honda Orthia had on the rear seats and sunroof, would have helped reduce the heat slightly but that the dark colour of the car could have increased the inside temperature by as much as 6C.

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White explained that a window wound down by 50mm would not be enough to cool the car but that if two windows were "cracked" and with sufficient wind it could drop the temperature by several degrees.

"To get greater cooling you have to have larger openings. So a window or door open entirely. So you get hot air leaking out of a big enough opening."

A slightly open sunroof would have much the same effect as a slightly open driver's side window, White said, but fully open it could trigger the spilling out of hot air and cool in coming in.

However defence counsel Julie-Anne Kincade pointed out White never visited the scene.

It was Neil who retrieved Isaiah from the car later that afternoon, possibly about 3.30pm.

He described the little boy as hot and lifeless however Te Whetu was not concerned when Isaiah did not take a bottle, believing he was in a deep sleep and putting him into his cot.

It wasn't until Te Whetu woke again at 6.30pm and discovered Isaiah not breathing that she raised the alarm.

The court heard earlier from Detective Ashley Clements who said when he arrived at the scene the morning after Isaiah died, the sunroof of the car was closed and had dust on it.

Clements said he could not initially see a car seat inside the station wagon because of the tinted windows.

The detective took photographs of the house at Ngāhina Rd and recorded details of the scene.

He told the court Isaiah slept in a portacot in a room by himself that had a broken window pane.

The cot had three blankets, a white fleece blanket or rug, and a fifth blanket covering the mattress.

A baby's water bottle sat beside the portacot. A bong for smoking drugs made from a can, was found in the freezer.

The trial in front of Justice Sally Fitzgerald will continue next week.