Fire and Emergency crews have mostly contained the blaze on State Highway 1 at Waipapa Bay, north of Kaikōura, in the Canterbury region.
Emergency services were called to the scene, where flames and billows of smoke were stemming from the truck's trailer, shortly after 2pm.
The highway was closed as crews from Kaikōura and Seddon fought the blaze.
The road had since been reopened with traffic management in place.
Christchurch man James Mackie, who had been traveling south with his family, told RNZ he first spotted fire in scrub alongside the highway before discovering the flaming truck.
"I jumped the fence and got into a paddock and got a side view of the truck.
"It was pretty hectic, I was standing about 50 metres away but the heat was unreal."
The truck cab was able to disengage from the trailer and the driver was uninjured, Mackie said.
Contents from the truck, such as smiley face balls, hula hoops and frisbees could be seen strewn across the road and shrub, he said.
"They were starting fires in the dry shrubs. The driver was unaware it was lighting fires ... that's what we noticed first."
Waka Kotahi/ NZ Transport Agencyt said motorists should expect delays to continue and to follow instructions of emergency services.
