The cause of a fiery car crash which killed New Zealand historian Michael King and his wife, Maria Jungowska, may never be known.

At an inquest in the Huntly District Court yesterday, Huntly Coroner Bob McDermott said the couple died from traumatic injuries sustained in the accident, which was most likely caused by driver inattention of some sort.

Their Subaru station wagon, driven by Dr King, left State Highway 2 near Heaven Rd in Maramarua, slammed into a tree and burst into flames.

The couple were on their way from their Opoutere home, north of Whangamata, to Auckland Airport to fly to Kerikeri.

The sun was shining, the road was dry and Dr King was driving with Ms Jungowska sitting in the back.

She often did that on long journeys, Dr King's sister, Geraldine Judkins, told the coroner.

It helped make the journey comfortable for her because she had multiple sclerosis.

German tourists Michael Wannhoff and his girlfriend Iris Kraemer were following Dr King's car, doing between 80km/h and 90km/h, before the crash.

"Next thing I know it drives across the other lane and runs across the road. I see two trees. I hope it drives between them," he told police.

The car hit the first tree head on.

Mr Wannhoff ran to the car calling out, but received no reply.

He saw Dr King's hand hanging out the window before the car burst into flames.

Mr Wannhoff tried to fight the fire with an extinguisher from his camper van.

"The fire was too big for me, there was nothing I could do."

Dairy farmer Graham Heaven told the coroner he arrived soon after with another extinguisher.

"I was aiming my extinguisher at the driver's side. It was getting hotter and hotter."

Pathologist Dr Frederick Mayall said Dr King would have been dead when the car caught fire.

Ms Jungowska's injuries would have been fatal, and she was probably unconscious at the time of the fire.

Neither had inhaled soot, which indicated they were not breathing when the fire started.

Dr King had recently completed cancer treatment and had been given the all clear. Dr Mayall said he could find no health conditions which could have left Dr King suddenly incapacitated.

Police reports said the road was good - not the case in some other crashes on State Highway 2, the coroner said.

And although the fire gutted the car, no mechanical failure or fault could be found.

Senior Constable Mark Chivers said there were no skid marks at the scene, and estimated the car was travelling between 73km/h and 93km/h when it hit the tree.

There was no evidence the crash was a deliberate action by Dr King.

Mr McDermott said the crash was most likely caused by driver inattention, as all other options had been ruled out, although the incineration of the car and bodies had made examination difficult.

"New Zealand has lost a great scholar and writer with the passing of Dr King, and it will be a terrible blow to his family and friends," he said.

In 2003 King was named the New Zealand Herald New Zealander of the Year, and was named a "giant kauri" in the Prime Minister's Literary Awards.

Dr King moved from journalism to literature in the 1970s, and had published 33 books, including the Penguin History of New Zealand.

The first print run of the Penguin History - 10,000 copies - was sold out in six days.