The crash about 2.30pm in the southbound lane closed the road.
Traffic detoured on to Middle and Lyalls Rds.
The highway was expected to remain closed until early evening, to give the serious crash unit time to complete its investigation.
The driver and a passenger in the stock truck were not injured in the crash "but very shaken", Senior Constable Clint Vallender, of Waimate, said.
He said the woman was driving her Nissan Sunny north.
She was the only occupant of the car.
Opposite the St Andrews Golf Club on a moderate bend, the car collided head-on with the empty stock truck, which was travelling south.
The Nissan was extensively damaged, and emergency services personnel had to cut away the roof and doors to reach the trapped woman.
Snr Const Vallender said the name of the dead woman would not be released until family had been notified.
One person died and another was seriously injured in an apparent head-on collision on State Highway 3 just north of Wanganui about 2.45pm yesterday.
