Sergeant Dougall Henderson, of the Southern District serious crash unit, said it was too early to say conclusively what caused the crash.
It appeared one car had crossed the centre line for unknown reasons and the other car had possibly tried to take evasive action, without success, he said.
The collision happened where no detour could take place and created a backlog of traffic, Sgt Henderson said.
The crash served as a reminder to remain alert and to concentrate on staying within your lane at all times.
It was also important to avoid distractions and drive at a speed appropriate to the conditions.
Extra speed significantly increased the chance of injury and death, Sgt Henderson said.
St John said four patients were taken to hospital with serious injuries and the other two people involved in the crash received moderate injuries.
Five helicopters were dispatched.
