Acting police sergeant Gareth Davis said the description was consistent with other eyewitnesses but it was too early to draw conclusions.
Davis said the family of five were driving south to Auckland when the accident happened. He said the roads were not busy at the time.
One West Coast Rd resident, who asked not to be named, said: "It just looked like he fell asleep. It's on a straight bit of road."
There was a worse crash further north, where a man died after he lost control of his vehicle along Kaitaia-Awaroa Rd, between Kaitaia and Ahipara.
Kaitaia Senior Sergeant Geoff Ryan said the man lost control on a moderate bend, broke through a fence and crashed into a paddock.
Meanwhile, a motorcyclist was airlifted to Nelson Hospital in a serious condition after a crash on the Whangamoa Saddle near Nelson yesterday.
The accident happened about 3.30pm near the summit of the Whangamoas when the 56-year-old male rider hit a bank.
And a heavily pregnant woman is in hospital after crashing into a lamppost, which then fell on to her car, narrowly missing two young children. Marcia Trembath and her children, aged 2 and 4, were trapped inside a crushed Hyundai Lantra station-wagon after skidding off Auckland's Northwestern motorway about 10am. Her partner Dean Delaney, a well known soccer player for Mt Albert-Ponsonby, told the Herald on Sunday said she had a fractured spine but there were no known injuries to their unborn baby.
"We are still in hospital so not doing the greatest ... she is 35 weeks [pregnant], and that is looking okay still," Delaney said.
Four people were killed in three separate crashes in the South Island on Thursday and Friday.
