The loaded concrete agitator was in the mine's decline and the operator had set up to spray cement when he saw a fire at the front of the agitator.
The fire was put out by the onboard fire suppression system and a hand-held extinguisher.
The cause of the fire was determined to be water ingress into the starter motor.
It turned out the truck had been modified and that allowed water, which was regularly used to clean the machine, to get into the engine bay and subsequently the starter motor.
The New South Wales Resources Regulator warned miners to be careful when modifying equipment.
"When modifying equipment, mine operators must conduct adequate change management to ensure any new risks are identified and additional controls are implemented as required," it said.
"In this case, modification of the engine covers to address another risk allowed excessive water into the engine bay."