The grandmother of a baby boy in New Zealand who tragically died after he was "cooked" inside a hot car while she went away to smoke synthetic cannabis with the child's parents has now been found guilty of manslaughter.

54-year-old Donna Catherine Parangi denied that she had any role to play in her eight-month-old grandson Isaiah Neil's death in 2015 but was found guilty on March 25. The High Court in Hamilton was told that Parangi had said in a police interview it was her idea to leave the child in the car so that he could continue sleeping.

The New Zealand Herald reported that Isaiah was sick in the days before he died and had been suffering from flu-like symptoms. Parangi had left the infant in the car while she went inside a residence to smoke synthetic cannabis with the child's parents, Lacey Te Whetu and Shane Neil.

The parents have already pleaded guilty to their son's manslaughter. The court introduced an expert to the trial who said that the infant died from a heat stroke, which is also known as hyperthermia, after he was left in the hot car. The New Zealand Herald reported that five adults had visited the house in total that afternoon but none of them were able to rescue the infant.

Childcare workers who dropped off Isaiah's two older siblings off in a van at 3 pm that day also said that they noticed the blue Honda but did not spot a baby inside. The court also heard a statement from family friend Paul Daniels, who said that he came to the home to pick up gardening equipment. He said he did not see if any of the windows were open or closed at the time and that he went back to the house later that night after he saw an ambulance arrive at the home after 6:30 pm.

Daniels said: "Shane was acting all weird. He didn't seem concerned with what was going on. Isaiah was in the back of the ambulance. Lacey and Donna were hanging around the back. They seemed in shock and upset. Santa was standing nearby and Shane was just wandering around."

Neil, who was also found guilty of his son's manslaughter after he admitted his role in the child's death, told the court that he had found Isaiah in the vehicle and unresponsive at around 3:30 pm. He was also one of the people smoking synthetic cannabis.

The father said he took his son and handed the baby over to Te Whetu who tried to give the child a bottle before she decided to put him down in his cot. She then tried to wake her son at about 6:30 pm but couldn't get the boy to wake up.

Stuff reported that Parangi was led from the courtroom after she heard the verdict and she allegedly scowled at the jury as she walked by. She said: "Thank you. So much." She has been remanded into custody and will be facing her sentencing on May 10 in Hamilton, New Zealand.