The PSNI in Down say that shortly before 8 pm on Thursday, 21 January, a two vehicle collision occurred on Newcastle Road, Dundrum, close to the entrance of Murlough Nature reserve.
A female pedestrian, aged in her 30s, was injured during the collision.
She was taken to hospital where she remains in a serious condition.
Inspector Stephen Humphries said: “We are investigating the circumstances surrounding the collision.
“I would like to hear from anyone who was in the area at the time and witnessed this incident. We can be contacted on 101, quoting ref: 977 210116.”