At the time of the incident the mine was undertaking maintenance activities on the methane drainage plant, resulting in the methane range being pressurised. 

The high potential incident resulted in methane being discharged in to the general body roadway of the mine.

As a result of the damage to the methane range, general body methane levels exceeded 2.0% with a peak reading being identified by a mining official of 3.5% at a workplace inbye of the incident scene while in the process of withdrawing workers from the area. 

As the incident progressed, there was then a failure to apply the mine’s withdrawal trigger action response plans, resulting in people remaining underground as methane levels increased.

Centennial spokesperson Katie Brassil said Mandalong mine's priority is the safety of its people in what can be a challenging underground environment. 

“We have worked closely with the regulator to ensure all our systems and processes reflect the highest of standards,” she said.

“Mine operators should consider change management processes being applied to work that may have a flow-on impact to other activities at the mine and engineering controls for the automatic shutdown of diesel plant should general body methane be greater than 1.25%,” Mine Safety NSW said in an alert.