Three children and a grandmother were pulled from a burning car by a hero who risked his life to save the trapped children because "they've their whole lives to go".

Real estate agent Rob Kuypers last night spoke of his grief at not being able to save Adam Pretorius, 4, of Milford, who died in the accident.

But police credit his actions - and those of a truck driver who also stopped to help - with saving two other children in the car and the woman driver in her 60s.

The survivors, along with the 17-year-old driver of the other car, are in hospital.

The Herald on Sunday was unable to contact Adam's parents David and Natasha Pretorius.

Kuypers, 52, was driving his son from school to their home in Dairy Flat when he arrived at the crash scene.

"It was on fire when I got there and the big thing ... was because of the fact that it was on fire no one wanted to go there."

By the time Kuypers had pulled the grandmother and oldest child from the front seats, the car was "pretty well ablaze".

"There were a lot of spectators watching it and didn't want to go near it because they thought it would blow.

"I thought, 'It's not going to blow before it gets to the tank'. The kids ... you just think 'stuff it, we're gonna get them out of there'."

He said the children in the back seat had seatbelts on.

"The first child, unfortunately I could already see he was dead. The other was pretty badly injured.

"I put them on the grass on the side of the road and checked the vehicle to see if there wasn't anyone else in there."

He then turned to help get the teenage driver from the other car.

Kuypers said the only thing running through his mind was "just get them out".

"It doesn't matter what happens. They're kids - I'm 50 years old. They've got their whole lives to go. I'll take the risk."

"To the family - I'm sorry I couldn't do much for the young one. I just hope that everything goes well for the rest of them. I really feel for the grandmother.

"I feel for the other girl in the car too. How she's been confronted with this whole thing will be hard.

"The time from when I stopped there, within five minutes the car was already burned out. It really surprised me how fast it burned."

Orewa acting Sergeant Natasha Hill said the boy was travelling south with his grandmother and two sisters, ages 6 and 9, on East Coast Rd in Redvale.

Police said the car carrying the children crossed the centre line, crashing into an oncoming car about 3.10pm.

Officers did not believe speed was a factor.

Ambulance northern communications team leader Ngaire Jones said five people were taken to North Shore, Auckland and Starship hospitals.

One patient was critical, one with serious injuries, and three others had moderate injuries.

One neighbour drove past the crash scene. She said: "I saw the two cars on the truck getting taken away. It was pretty bad."

The woman said the crash happened on a dangerous stretch of road.

- additional reporting Rebecca Lewis