Two children are dead and one has internal injuries after a shocking crash near Dubbo in the early hours of Saturday.

Emergency services were called to the crash site on the Newell Highway, about 12 kilometres north of Dubbo, about 3.30am after a 4WD and a B-double truck collided.

A couple were travelling with their three children in the four-wheel drive at the time of the crash.

Two boys, a nine-year-old and a 12-year-old, were found dead at the scene. A third child, a five-year-old boy with internal injuries, was flown to Westmead Children's Hospital in a serious condition.

The parents of the boys suffered minor injuries and are under observation in hospital, where they are being treated for shock.

"All support is being offered to that family during this unfortunate tragedy," Orana Local Command duty officer Jason Pietreszka said.

"The causes of the crash are under investigation."

The male truck driver suffered serious injuries and was also flown to hospital.

The crash left tree branches strewn across the highway. The truck appears to have come to a stop in scrub on the side of highway, close to a bridge and dozens of metres away from the four-wheel drive.

The NSW police crash investigation unit has travelled to the scene from Sydney and are trying to ascertain the cause of the crash.

Traffic along the highway was blocked in both directions and motorists are advised to use an alternate route as the Newell Highway remains closed.

Officers will prepare a report for the Coroner.

Daily Liberal