Gwent Police is continuing to appeal for witnesses following a crash on the A48 at Pwllmeyric, near Chepstow in the early hours of Saturday morning.
During the incident a black, Honda Civic collided with a wall.
A spokeswoman for Gwent Police said: "Following the collision, we believe a second vehicle, a Ford transit van, unrelated to the first incident has collided with an occupant of the first incident outside of the damaged black Honda Civic.
"A 32-year-old male from Newport was seriously injured and is currently receiving treatment at the Heath Hospital, Cardiff.
"The unidentified vehicle failed to stop at the scene.
"A female from the Newport area was subsequently arrested in relation to the initial collision and has been bailed to appear at court.
"The identity of the vehicle and driver involved in the subsequent collision are still unknown. Enquiries into both incidents are ongoing.
"Anyone who may have witnessed the collision or has any information on the incident or the driver of the transit van is asked to contact Gwent Police on 101 quoting log 44 17/02/18."
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