The neighbour of the house where baby Isaiah Neil was left in a parked car saw his two older siblings playing in the stationwagon the day he died.

Sonia Biddle has told the High Court at Hamilton, where Isaiah's grandmother Donna Parangi is on trial for the baby's manslaughter, she remembers the two preschool-aged children playing in the car when they returned from kōhanga reo that afternoon.

It was during the three hours Isaiah was left in the car on November 2, 2015, and 4 1/2 hours before he was pronounced dead.

However, Biddle said she did not remember previous testimony she gave two years ago that she also saw Isaiah's father Shane Neil collect the children from the car.

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"I've been hospitalised for memory loss in the past four years so I can't remember some things."

The testimony suggests Neil was aware Isaiah was in the car but left him there for another half an hour.

The Crown says Parangi is culpable over the 8-month-old baby's death because she cared for her grandson and his two older siblings who lived with her, partner Santa Te Whetu, and parents Lacey Te Whetu and Neil in Rūātoki.

The Crown says Parangi, 54, and Lacey Te Whetu left the baby in the car after arriving home from a drug run to Kawerau.

Isaiah Neil was just 8 months old when he died inside a hot car in November 2015. Photo / Supplied
Isaiah Neil was just 8 months old when he died inside a hot car in November 2015. Photo / Supplied

The mother and daughter had bought synthetic cannabis from a cousin, and went inside leaving Isaiah to sleep in his car seat.

Together with Neil the trio smoked the synthetic cannabis and fell asleep.

Lacey Te Whetu and Neil have previously admitted manslaughter over their son's death.

Biddle, who knew the family, spent that "hot" spring day weeding her garden, unaware Isaiah was just metres away in the parked car.

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She said the doors and windows on the car closest to her house were shut but that the sunroof was open.

During the three hours Isaiah was in the car, up to five other adults who did not live at the house in Ngāhina Rd, visited the property and saw the car but no one noticed the baby.

They included Te Whetu's brother Marcus Te Whetu who washed his car on the property that afternoon, and had visited the house in the morning to check on his nephew whose welfare he said he was worried about.

Two kōhanga reo van drivers dropped off Isaiah's older siblings, an acquaintance arrived to collect a weedeater, and Marcus' then girlfriend Felicia Riini said she was also there briefly while Marcus collected something.

Neither they nor Biddle knew Isaiah was in the hot car while the four adults inside the house, including Santa Te Whetu, thought to check on the baby.

Isaiah was eventually retrieved from the car by his father at 3.30pm, hot and lifeless, according to Neil, and when the baby would not take a bottle, Te Whetu thought he was in a deep sleep.

She put him in his cot before returning to her slumber for three hours. At 6.30pm, when she couldn't rouse Isaiah, she rushed next door to Biddle's house and tearfully asked to use her phone.

Biddle said she didn't ask why. Te Whetu raced home and rang emergency services.

Biddle's daughter Sharon Biddle said she raced to the property when asked if she could perform CPR.

Sharon recounted the moment she saw Isaiah in a bedroom on the night he died.

"There was a bed and his mum. They all just tried to help, with CPR and talking on the phone with the ambulance."

The witness was visibly upset at the memory prompting Justice Sally Fitzgerald to offer her a break.

Sharon Biddle also said she saw Neil sitting on a step outside the house that afternoon watching his older children play outside.

Earlier in the trial Parangi's defence counsel Susan Gray pointed to asphyxiation or abuse as the cause of Isaiah's death, not heatstroke.

The three-week trial, being heard in front of a jury of eight men and four women, continues.