Seat belts were not being worn in the car that crashed near Taupō yesterday killing five members of the same family, police say.
An 11-year-old boy was the sole survivor of the crash near Kinleith, 30km north of Taupō.
He was taken by helicopter to Waikato Hospital, with serious injuries, and is in a stable condition.
Taupō area road policing manager Senior Sergeant Fane Troy said at this stage excessive speed did not seem to be a factor in the crash.
Police were still working out why the car left the road, where it collided with three trees.
"Unfortunately people were not wearing their seat belts in the vehicle and that has led to the deaths," Mr Troy told Morning Report.
The family was in a five-seater vehicle and the 11-year-old was believed to have been in the back seat with three other young people.
"At this stage from our early investigation it appears that nobody in the vehicle was wearing a seat belt.
"Unfortunately when people are not restrained, or items are not restrained in a vehicle, when you have a crash, those items become missiles and they do all go forward."
A relative was flown with the injured boy from the scene to Waikato, Mr Troy said.
He said the weather conditions were dry when emergency services got to the scene but investigators are still to determine the time of the crash.
Two other fatal accidents occurred yesterday - one in Ashburton that killed three people and another in Auckland that left one person dead.