His grandmother Donna Catherine Parangi is on trial in Hamilton for manslaughter. Te Whetu and Neil previously pleaded guilty to the charge.
The trio smoked synthetic cannabis on the day Isaiah died and fell asleep, leaving the baby strapped into his car seat for three hours on the warm spring day.
When Renwick arrived at the scene that night he was warned by paramedics that the 8-month-old baby's death might be suspicious because of bruises found on the infant.
He treated the house like a crime scene and recorded everything he saw and heard.
"What we knew about how that baby died was very limited. I could see the mother was extremely distraught with what was happening, so it wasn't appropriate to remove anyone from the scene."
At 8.19pm Te Whetu attempted to wake Neil, who had gone back to sleep after his youngest son died.
She shook Neil's leg but Renwick said the father "opened his eyes, said something I couldn't make out and appeared to go back to sleep".
Te Whetu continued to hold Isaiah, wrapped in a blanket, and moved to get her two older children warm clothes.
"I saw her unwrap the baby. There was a bruise on the left rib of the baby. I saw her look at the bruise and let out a little sigh."
Renwick described the bruise as linear in shape, about 10cm long across the left rib.
"She re-wrapped the baby, was cradling the baby and crying."
Renwick obtained a statement from Neil but said he was drowsy throughout.
"He appeared to sleep at one point while we were talking. I asked him if he had had anything to drink that day and he said 'No'.
"I asked what he was doing generally and he struggled to give me any detailed response. At one point he asked me what had happened to baby."
Renwick said Neil "zoned out" and forgot what the pair were talking about. He took Neil to the police station and said at one point he re-entered an interview room and Neil was again asleep.
Earlier in the trial Parangi's defence counsel Susan Gray pointed to asphyxiation or abuse as the cause of Isaiah's death, not hyperthermia - known as heatstroke.
Crucially Te Whetu testified that she was not concerned by Isaiah's condition when Neil retrieved the baby from the car and handed him to her at 3.30pm.
When Isaiah wouldn't take a bottle she assumed he was in a deep sleep and put him in his cot.
At 6.30pm, when she awoke again and couldn't rouse Isaiah, she called 111.
Paramedic Linda English testified that Isaiah was warm when she arrived at the house at 6.52pm indicating he had died recently.
The court also heard the police statement taken from Parangi on the night of Isaiah's death in which she omitted the fact she, Te Whetu and Neil smoked the synthetics that day.
Parangi also lied in the statement, saying that Isaiah was taken out of the car by Te Whetu about 12.30pm when she and her daughter arrived home from Kawerau.
She said Te Whetu gave Isaiah lunch and put him to bed, but the jury of eight men and four women already know Isaiah was left in the car that afternoon.
The trial is set down for three weeks in front of Justice Sally Fitzgerald.
