Speed and alcohol were ruled out.
Acting Sergeant Brett Steel said the driver of the Rav 4 may have fallen asleep from fatigue.
He said luckily no one was hurt because the stretch of road was often busy with all manner of vehicles, including logging trucks.
Police closed a northbound lane but not for long while the vehicles were moved to the side of the road.
Shortly after that accident, a ute heading south veered off the road and went down a bank before rolling several times in a paddock.
Steel said the sole occupant, a Whangārei man, was not hurt in the crash on State Highway 1 just north of the Mangapai Rd junction.
The ute was a write-off.
Steel said people should not let fatigue get the better of them.
"If people feel tired, they should stop their vehicles even if they are close to their destination, get out and walk around for about five minutes before carrying on."