The hydraulic shovel was set up to change its rollers.
Two stands were put in position to support the shovel.
Additional repair works to the bucket were then identified, which required the shovel to be moved.
While repositioning the bucket, the shovel became unbalanced and slid off the support stands.
The NSW Resources Regulator said the stands were pushed into two load rollers, breaking their mounting bolts and causing them to become dislodged.
"When a machine is sitting on support stands and must be operated, a risk assessment should be conducted and additional controls required put in place prior to proceeding," it said.
"When planning multiple maintenance tasks on a piece of equipment, risk assessments must be undertaken to ensure that workers carrying out one task do not introduce risks to workers undertaking other tasks."