Golden Grove Operations Pty Ltd has been fined $35,500 after an integrated tool carrier and blast truck fell into a 13-metre deep hole at the company's Golden Grove precious metals mine southeast of Yalgoo in October 2018.
While no one was injured in the incident, three workers from a blast crew had been working in close proximity to the bomb truck minutes earlier.
A number of charged blast holes were also engulfed in the sinkhole.
Investigations found the root cause of the incident was backfilled underground workings had not been treated as a void, allowing work to be undertaken above an area of 'unknown stability'.
Acting WorkSafe Chief Inspector Mines, Christina Folley said Golden Grove should not have allowed workers to operate above a previously filled void where the ground's stability had not been verified.
"This was a near-miss that could have resulted in serious injury or loss of life," Ms Folley said.
The workers who had been in the area shortly before the incident were lucky to escape safely."
"Voids pose a significant workplace hazard, so mine operators must develop safe working practices that do not presume any stope is tight-filled."
Between July 2018 and October 2018, the company failed to undertake probe drilling to confirm the site's safety. Nor did it apply the Working Around Voids Procedure to mining in that area.
Mining operations at the incident site were discontinued and have yet to resume with further geotechnical reviews of all the stopes being conducted.
Ms Folley said geotechnical hazards are well-recognised in the mining industry and that mine operators must monitor and record any change in underground conditions below open cut workings.
The company, then known as EMR Golden Grove Pty Ltd, pleaded guilty in the Perth Magistrates Court for failing to provide a safe work environment, and was ordered to pay $5000 in costs.
A second company Minjar Gold Pty Ltd has also been charged in relation to the incident and will appear in court at a later date.
Minjar previously mined ore at the precious metals mine through an arrangement with the former owner. The company entered into an agreement with Golden Grove and was mining ore from Gold Pit 3, the site of the subsidence.
