Police are appealing for witnesses to a fatal crash on State Highway 7 near Greymouth on Saturday evening.

One person was killed and three others were injured in the two-car crash which occurred in the Arnold Valley just after 6.15pm.

Police said initial indications are a southbound car crossed the centre line, colliding with a car coming from the north.

The driver of the southbound car was thrown from the car. Police said he was airlifted to Christchurch Hospital, where he is reportedly in a critical condition.

Police said a 56-year-old woman, the driver of the northbound car, died at the scene.

"Two teenage passengers in the northbound car suffered serious but non-life threatening injuries and were taken to hospital for treatment," a police spokesperson said.

"Police are working to support the families of all those involved in the crash."

The road reopened early on Sunday morning after a scene examination was completed overnight.

An investigation into the cause of the crash is ongoing.

Anyone who witnessed the crash is asked to call 105, quoting event number 220501/3591.