Trustpower TECT Rescue Helicopter spokesman Liam Brettkelly said the family, believed to be from Kaitaia, were driving to Gisborne.
Mr Hendl described the scene as chaotic.
"[The father] made efforts to get into the car - smashing windows."
A passing doctor immediately started helping with the critically injured baby, working on it for an hour before travelling with the infant to Whakatane Hospital's emergency department, he said.
"He was obviously a great help. He worked intensively on the poor baby and did a fantastic job."
The baby was transferred to Auckland's Starship hospital but died there on Sunday night.
The doctor, who works at a Gate Pa medical centre, did not want to speak to reporters.
The baby's 4-year-old sister was flown to Auckland City Hospital, where a spokesman said she was in a stable condition.
The children's 26-year-old mother, who was sitting in the back seat with the children, was also injured and was in a stable condition in Auckland City Hospital.
Eastern Bay of Plenty ambulance territory manager Richard Waterson said the grandmother, who was trapped in the car and had to be freed by firefighters, suffered leg injuries.
A Whakatane Hospital spokesman said she was in a stable condition. The truck driver was not seriously hurt.
Mr Hendl said the crash happened 100m from the scene of another fatal crash last month and the area was a known black spot. In January a man in his 20s died when his car left the road and hit a tree.
Mr Hendl appealed to other drivers to take care in the area.
"Please make sure you're refreshed and pay attention and drive safely."
