The incident happened on the Moira Road in Glenavy on Friday just before 3 pm.
The PSNI’s Collision Investigation Unit based at Steeple Barracks in Antrim is probing the crash.
The road has been closed and is expected to remain closed for some time.
Diversions are in place at the junctions of the Ballydonagh Road and Garlandstown Road.
Police have appealed for anyone with information or who may have witnessed the crash to contact investigating officers at Steeple Barracks.