Detailed in a safety alert, the machine was found with a one-quarter-inch British Standard Pipe plug missing from the water-cooled exhaust pipe.

This plug is removed to enable raw gas testing, and raw gas tests of the machine had taken place in the previous week.

“The test port plug may have been insufficiently secured after testing or may have been, inadvertently, overlooked,” the inspectorate said.

Consequently, the diesel engine exhaust gases from the machine were able to enter the underground atmosphere without going through an exhaust scrubber.

“The explosion-protection properties of the engine were diminished while it was in an Explosion Risk Zone 1 – a highly dangerous situation,” the inspectorate said.

“The Queensland Mines Inspectorate has become aware of instances where the integrity of the explosion-protection properties of diesel engine systems used in underground coal mines has been compromised.

“In this incident, it is highly probable that a machine had been operating within an ERZ1 without a test port plug, which means that exhaust gases would have been exiting the engine directly into the mine atmosphere.”

The inspectorate wants all underground coal mines in the state to review the relevant procedures for maintaining and testing explosion-protected diesel engine systems, adding that original equipment manufacturers should be included in these reviews to assist with possible engineering solutions.

In the meantime, the inspectorate recommended that all senior site executives audit their underground diesel fleets.