Gap this morning.
Investigators have been told the Hyundai sedan was travelling north west on the Ararat Halls Gap Road about 6.30am when it struck the tree, about 2km south east of Grampians Road, Victoria Police say.
The woman, who was seated in the rear passenger seat, died at the scene.
A man, also seated in the rear passenger seat, was airlifted from the scene with serious injuries.
The male driver and male front seat passenger were transported to a hospital with non-life threatening injuries.
Major Collision Investigation Unit detectives are en route to the scene.