The rear of the truck caught fire about 8.30pm (AEDT) while travelling along the New England Highway about 30km south of Tenterfield, in the state's north, a NSW Fire Brigades spokesman said.
About 40 firefighters were trying to contain the blaze.
The spokesman said the truck was carrying a hazardous material, but he could not reveal the nature of the substance.
The truck driver had disconnected the rear trailer from the rest of the rig, he said.
The New England Highway was closed in both directions at the accident site.
There were no reports of injuries and the fire was not threatening property.
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