A Caterpillar 789C was being commissioned after a rebuild.
Four workers were investigating a transmission fault when the transmission engaged unintentionally propelling the truck forward about 1.5m.
The truck stopped when one of the workers applied the service brake.
The NSW Resources Regulator said live testing protocols were in place and all workers on the task were onboard the truck.
"This incident demonstrates the need for a safe system of work to be in place when conducting function testing," it said.
"This system should limit workers to a bare minimum required for the testing. Additionally, no-go zones and barriers must be in place and enforced to prevent other workers from entering the work area."
