Leading Senior Constable Justin Rasmussen, of Mortlake police, said the driver of the Cobden vehicle had noticed smoke in the cabin around 5am on Saturday morning.

The driver pulled over and on the Darlington-Camperdown Road, about three kilometres south of the Hamilton Highway, and separated the truck's trailer from the vehicle.

A Country Fire Authority spokesman said crews from Bookar, Darlington, Derrinallum and Wooriwyrite extinguished the blaze.

Senior Constable Rafmusse said the driver was not injured in the fire and the truck's trailer was not damaged.

"The truck was destroyed," he said. "The vehicle had been serviced and was well maintained.

"Trucks do catch on fire unfortunately, the brakes malfunction all those sorts of things.

"Drivers are trained to do separate the trailer; if the truck catches on fire, save the cargo."

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