Bunn said everyone involved had been wearing a seatbelt.
Traffic volumes are expected to swell as summer holidaymakers hit the region's roads for a much-needed break north.
With that in mind, Bunn reminded motorists to drive considerately.
"If there's heaps of traffic out there you've just got to accept you have to go with the flow."
He urged people travelling below the speed limits to be considerate and allow other vehicles to pass when and where appropriate.
"... as a driver not only are you responsible for your own vehicle but also for what's in your environment," Bunn said.
"As in, what's behind me? Am I going too slow? Do I have 20 cars behind me, maybe I should pull over. If it's raining and wet, adjust your driving, keep your spacing."
Thursday's fatal collision takes this year's road toll to 31, ahead of last year's total of 29 road fatalities.
Bunn asked anyone in the area who may have seen the Waipū crash or the vehicles driving beforehand to contact police on 105 and quote file number 211223/3317.