The truck driver, who was shaken but not hurt said he had seen the car overcorrect "quite aggressively" immediately before impact, Mr Kitchen saying it was possible that the driver had fallen asleep.
The truck continued south for some 50 metres, in the drain, the driver saying he was surprised that it had remained on its wheels.
Debris from both vehicles was scattered over more than 50 metres of highway, the truck losing its diesel tank. Mr Kitchen estimated that some 400 litres was spilled, along with oil and transmission fluid, before a Fulton Hogan crew laid sand and the highway was finally reopened, more than two hours after the crash. (Traffic was diverted via Ruaroa and Takahue roads).
Kaitaia Fire Brigade station officer Ross Beddows said the fact that the car's engine block was thrown 10 metres gave a good indication of the force of the impact. Firefighters had investigated after noticing a gouge in the bank, some flattened grass and three broken fence wires.
"It neatly bisected two large tree trunks before landing in long grass another three metres away," Mr Beddows said.
"You don't need much imagination to understand what would have happened to anyone unlucky enough to have been in its path."