Police are investigating after a 10-year-old boy died and two people were injured when a car reportedly hit a tree on a rural road in the NSW Hunter region.

About 11am Saturday, emergency services responded to reports a vehicle left the road and hit a tree before it rolled a number of times on Flaggs Rd in Merriwa, about two hours northwest of Mudgee.

The boy, a passenger in the car, was treated by paramedics but died at the scene.

A 15-year-old girl was taken to John Hunter Hospital in Newcastle in a stable condition.

The male driver, 46, was flown to the same hospital in a critical condition.

Hunter Valley Police are investigating the crash and have urged anyone with dash cam footage to come forward.

A report will be prepared for the coroner.