The Crown has closed its case against a Ruatoki woman whose infant grandson died after being left in a hot car.

Donna Catherine Parangi, 48, has pleaded not guilty to the manslaughter of Isaiah Te Rangi on November 2, 2015, by failing to provide him with the necessaries of life and failing to take reasonable steps to protect him.

Isaiah's parents Lacey Te Whetu and Shane Neil have pleaded guilty to his manslaughter.

Parangi was not in the High Court at Rotorua today to hear medical evidence against her.

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Justice Graham Lang excused her after defence lawyers told him she didn't want to be present while it was given.

In a video interview played to the jury, Parangi claimed she'd left the infant asleep in the family station wagon on a hot day with the windows down, the doors and sunroof open.

The Crown claims he was there at least three hours. Evidence has also been given that at least one of the car's electric windows couldn't be operated.

Waikato Hospital paediatrician Dr John Newman told the jury if Isaiah wasn't already dead when he was taken from the family's boiling hot station wagon, he would have been in severe trouble and in urgent need of hospital treatment.

Based on Ruatoki's temperature of 19C on the day Isaiah died, Dr Newman said the temperature inside the vehicle would have risen rapidly, likely reaching 45C within 45 minutes. He said this would have been the case even if the windows and sunroof were slightly open.

Testifying from Melbourne the pathologist who conducted the autopsy on Isaiah's body confirmed his death was heat-related but she was unable to determine when he died.

Dr Joanna Glengarry described finding "injuries of concern", listing these as bruises and abrasions on the infant's stomach, an ear and his right arm, saying indications were the latter two could have been bite marks.

She dismissed the possibility Isaiah could have caused the ear injury himself as Parangi claimed in her police interview. Questioned by defence lawyer Julie-Anne Kincaide, she was unable to say whether Isaiah had been alive or critically ill when he was taken from the car into the house.

Dr Glengarry disagreed with a suggestion his injuries were subtle.

"They were more than subtle," she said.

The trial resumes at 8.15am on Monday, timed so a London-based pathologist can give evidence by video link for the defence.