A vehicle involved in a double-fatal crash near Ashburton on Monday had been reported for erratic driving by other motorists before the crash.
Two drivers were killed when a small truck and a car - reportedly a people-mover - collided between Timaru and Ashburton shortly before 1.40pm.
The crash happened on State Highway 1 at the intersection with Ealing Rd just north of the Rangitata River.
A truck driver told Stuff he reported the car's erratic driving to police about 25 minutes before the crash, after seeing several near misses. At least two other motorists also complained, saying a vehicle was "weaving dangerously" through traffic moments before the crash.
The truck driver told Stuff he was driving north behind the car from Timaru until the collision. He said the car would slow at times to 40kmh before speeding up to 60kmh and 80kmh, then slowing down again.
He described the driver as a middle-aged woman who looked ``zoned out''. Stuff says it is thought she had suffered a medical event.
The driver said he told police there would be a crash if the driver was not taken off the road.
The call operator told him someone had been dispatched, but it was too late.
Emergency services called to the scene said they encountered a vehicle so badly mangled, "you couldn't even tell what kind of car it was".
One witness at the scene said the car flipped upside after the collision.
Senior Sergeant Phil Dean, of Ashburton, said the Serious Crash Unit was working to determine the cause of the crash.
The two drivers were the only occupants of their vehicles and both died at the scene.
-Additional reporting from NZME
