The Griffith driver, who asked not to be named, changed his plans at the last minute to sleep at the motel instead of bunking in the cabin overnight.

The prime mover was engulfed by flames in the Swaggers Motor Inn car park in Yass when firefighters arrived about 4.30am on Thursday, Captain Scott Lang, of Fire and Rescue NSW, said.

The driver said he had lost valuables including his log book, tools and mobile phone in the cabin fire.

But he counted himself lucky to be alive.

He said he had driven the truck from Griffith to Yass for its owner. The truck owner arrived to see the damage about 10am.

The motel's management said people driving past had noticed the cabin on fire and called emergency services, and that it was "a bit of a shock".

Firefighters contained the blaze to the prime mover and extinguished flames within an hour, but then had to manage 800 litres of diesel that had been compromised by the fire and was leaking from the truck, Captain Lang said.

Goulburn Fire and Rescue NSW's hazardous materials response team and Yass Valley Council helped Station 511 crew contain the spill to prevent the fuel from leaking into waterways, Captain Lang said.

The NSW Environment Protection Authority was on the scene while Castor Street was closed.

The council's engineering director Stan Robb said staff were using sand to soak up the diesel and stop it leaking into the drains before expected rain.

Police were not viewing the fire as suspicious, but possibly a fault with the truck, acting Inspector David Cowell of Yass Police Station, said.

There have been five truck fires, including one fatality, in Yass Valley since November.

The National Heavy Vehicle Regulator launched an investigation into truck fires in March this year following a spate nationally.