Authorities were alerted to the incident at about midnight on Tuesday, with reports of a B-double truck on fire nine kilometres south of Wellington on the Mitchell Highway.

"The information we got was a truck on fire and the cargo was nitrogen fertiliser, which if it gets alight can let off toxic fumes," Orana Rural Fire Service (RFS) Superintendent Lyndon Wieland said.

"But thanks to a quick response from the RFS, who got on scene very quickly, the cabin of the truck was lost but the cargo was saved."

Fire and Rescue NSW personnel from the Wellington Fire Station responded with hazmat support, but "after investigation on arrival at the scene it was determined that the scene was not a hazmat incident," Wellington Fire Station Commander Mark Moroney said.

Orana Local Area Command (LAC) duty officer Mark Dixon said the fire was caused by a "some break drums overheated which caused some material around the wheel arch to catch fire".

Both lanes were closed to traffic for approximately two hours, after which time it was deemed safe enough intermittently reopen one lane to traffic.

RFS personnel remained monitoring the scene until about 4.45am Wednesday, when both lanes fully reopened.