Donna Catherine Parangi, 54, had denied a charge of manslaughterof Isaiah Neil after he was left in a car while she and her daughter, Lacey Te Whetu, went into their house and smoked synthetic cannabis.
Isaiah was discovered in the car by his father, Shane Neil, who had also been smoking the substance.
Neil and Te Whetu have already admitted charges of manslaughter in relation to their son's death.
However, Parangi had denied any role in the baby's death.
The crown said Parangi had told police in her formal interview that she had told her daughter to leave Isaiah in the car when they returned so that he could continue sleeping.
From left, Donna Parangi, is currently on trial in the High Court at Hamilton. Her daughter, Lacey Te Whetu, and her partner Shane Neil have already admitted charges of manslaughter. Photo / File
The pair, along with Neil, then smoked synthetic cannabis.
The Crown's experts gave evidence that the baby died from heatstroke, or hyperthermia. Neil gave evidence about finding the boy lifeless and hot in the car and bringing him inside the house and handing him to his partner about 3.30pm.
She then tried to give it a bottle before putting him to sleep in his cot.
He then didn't wake up when Te Whetu tried to raise him about 6.30pm.
Parangi was remanded on bail until a nominal sentencing date of May 10.
Family friend Georgina Turanga, who was standing with Parangi's husband, told the Herald outside court the family was absolutely devastated at the outcome.
She said there were still a lot of unanswered questions that were important to the family.
"At the moment we are just going to look forward to having her home."
