Two toddlers have survived a single-vehicle crash that tragically killed their mother in western NSW.

Two passersby discovered the eight-month-old twins on Saturday afternoon, in the rear of a car that had rolled on the Silver City Highway at Anabranch, north of Wentworth.

Their 29-year-old mother Cody Marie Mitchell was found dead nearby.

Police say the infants were uninjured, with the witnesses releasing them from their car seat capsules and caring for them until emergency services arrived.

Officers are investigating the crash and a brief will be prepared for the coroner.