Emergency services were called to the highway, near Bargo just after 2:30pm on Monday.

Fire fighters were confronted with plumes of smoke with the truck well alight, a Fire & Rescue NSW spokesperson said. 

Drivers are urged to avoid the area with all southbound lanes closed near the Old Hume Highway. Traffic is backed up 6km.

There are no reports of injuries, but NSW Ambulance are assessing a patient. 

It's understood the truck is carrying fibre glass and pool materials. Three fire trucks from the NSW Rural Fire Service and Fire & Rescue NSW are on scene, and more have been deployed. 

More to come.