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

Two passersby discovered the 15-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 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.

In a separate crash, a woman and two children are in a critical condition after a car and a truck collided at an intersection in the NSW Riverina region.

Emergency services were able to free the 29-year-old woman and two girls, aged ten and four, from the car after the accident on the Snowy Mountains Highway about 3pm on Saturday and treated them for serious head injuries before they were airlifted to hospital.

The truck driver, a 58-year-old man, was not injured and was taken to Tumut Hospital for mandatory testing.