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

Officers from Orana Local Area Command attended and established a crime scene, while traffic along the highway was blocked in both directions.

Five people were travelling in the 4WD with two boys, a 12-year-old and a 9-year-old, found dead at the scene.

An adult male and female from the vehicle were treated at the scene for minor injuries but not conveyed to hospital.

A third child, a 4-year-old boy with internal injuries, was airlifted to Westmead Children’s Hospital in a serious condition.

The male truck driver and only occupant of that vehicle was treated at the scene for serious injuries and was airlifted to hospital.

Members of the NSW Police Force’s crash investigation unit travelled up from Sydney and are at the scene trying to ascertain the cause of the accident.

Motorists are advised to use an alternate route as the Newell Highway remains closed.

Officers will prepare a report for the Coroner outlining the circumstances surrounding the deaths.