A new tale of courage has emerged in the aftermath of a fiery accident which claimed the life of a 4-year-old boy just north of Auckland.

Adam Pretorius's family are grappling with the Milford youngster's death as his 6-year-old sister, Holly, prepares to undergo about nine hours of surgery today on her spine.

Another sister, Alex, 9, has been discharged and the children's grandmother, who was driving the car, is stable.

Police say their car collided with another vehicle on East Coast Rd, Redvale, about 3pm on Friday. An investigation will be carried out to determine whether charges should be laid.

John Johnson, who works for NZ Driver Licensing, was conducting a driver's test on the same road and came across the scene of the crash.

In the confusion of the aftermath, stories differ and Mr Johnson's version conflicts with that of estate agent Rob Kuypers, who was also at the scene.

Mr Johnson said he and the man he was testing ran to a woman in one of the vehicles. "Her head was back, her eyes were closed and she was bleeding but she was moving, so she was still alive."

They then turned to the car Adam Pretorius was in, which Mr Johnson said had small flames under its crunched bonnet but was not ablaze.

He said Alex was calling for help to get out of the front passenger seat, but wasn't crying. "It was amazing, actually. She is an amazing kid."

Mr Johnson said the children's grandmother was lying on the grass.

While he kept Alex calm and helped her get out, his driving student lifted Adam from the back. He said Mr Kuypers lifted Holly out.

Mr Kuypers told the Herald he pulled both children from the back seat and laid them on the grass nearby.

Mr Johnson said that about that time, another woman, who was medically trained, arrived at the crash scene. When the car began smoking more, the rescuers moved the children further from it.

Mr Johnson said he carried Adam in his arms and believed he was unconscious, but still alive. He then laid him on the ground, where the woman gave him mouth-to-mouth.

"She said, 'I've got a pulse'," Mr Johnson said.

"I just want the parents to know their little boy was actually alive and there was a lady who tried to help him, and their daughter was so brave."

Yesterday, a close friend of the Pretorius family, Martin Abert - speaking from the Starship children's hospital in Auckland - said parents David and Natasha could not cope "with more than one hour at a time".

"They really need to get through [Holly's surgery today], quite frankly, before they can even start focusing on Adam's funeral."

Mr Abert said messages of support had been flooding in from family and friends.