The Queensland Mines Inspectorate said the stacker boom was slewed and lowered, and contacted the stockpile.

"During this action, bogie wheels were partially derailed and the machine power tripped," it said.

 The front bogie came off the rail and moved about 250mm sideways when machine power was reset and the stacker boom slewed back into the park position.

"The control room operator was undertaking other production tasks at the time of the incident and did not stop the machine prior to the derailment occurring," the inspectorate said.

"The investigation identified PLC [programmable logic control] system design defects."