His baby died following the crash, and his 4-year-old daughter was seriously injured.
The car was being driven by the children's 58-year-old grandmother when police said it drifted across the centre line and hit the light truck on Thornton Bridge near Whakatane just before 2pm on Sunday.
The children's father was driving behind the vehicle and saw the tragedy unfold, Edgecumbe acting fire chief Roddy Hendl (EDS: crct) said.
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 on the critically injured baby, working on the child for an hour before travelling with the youngster 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," Mr Hendl said.
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 the media yesterday.
The baby's 4-year-old sister was flown to Auckland 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 also in a stable condition in Auckland 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.
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."
Sergeant Ray Wylie yesterday said initial enquiries indicated the car crossed the centre line into the path of the oncoming truck. The serious crash unit is investigating.
The driver of the truck was not seriously injured and had been discharged from Whakatane Hospital, he said.
- Rebecca Quilliam of APNZ