The Scott Simpson Contracting truck had a full load of big-bale wheat straw on board when it caught fire just after 2pm, apparently ignited by a spark from the stack exhaust.
Weston man Peter Ferris stopped to let the truck turn into Airedale Rd beside the Weston School, and noticed a "wee fire" beside the exhaust.
"I followed the truck on the wrong side of the road, flashing my lights trying to get him to stop," he said.
As the truck continued, the fire gradually spread along the rest of the bales.
The truck finally halted about a kilometre up Airedale Rd, where it intersects with Parklane Pl, when the fire in the straw "exploded", he said.
The driver tried to turn around, apparently intending to head back to the Weston Fire Station, but ended up partially blocking Airedale Rd before he got out of the cab and the fire brigade was called at 2.17pm.
Units from the Weston and Oamaru fire brigades, along with the Weston tanker, attended the blaze.
Because the fire was embedded in the straw, all firefighters could do was hose down the load until two tractors with hay forks arrived and started to push the load off the truck.
An attempt was made to tow the truck using the two tractors into a neighbouring paddock, away from under power lines, but the truck's brakes had locked.
The straw was pushed from the truck about 3.15pm into a heap on the side of the road and turned over so firefighters could put out the fire.
The brigades returned from the fire about 4.40pm.
