The maximum level found was believed to be 2.7%. 

Mine Safety NSW said a number of longwall tailgate methane levels were also detected at other mines at greater than 2%. 

It said the gas exceedances resulted from decommissioning booster fans along with a falling barometer and the distribution of ventilation in roadways in the vicinity of affected seals being inappropriate to adequately dilute gas from some of the seals. 

“Personnel must be withdrawn consistent with trigger action response plans,” Mine Safety warned.

Falling barometric pressure along with the longwall shearer in the tailgate zone for extended period caused goaf flushing, it said. 

“Production activities should be appropriately monitored and modified where increasing methane trends are detected,” according to Mine Safety.

“This can be achieved through the application of appropriate trigger action response plans and implementation of documented cutting sequences into the tailgate roadway.

“Ventilation officers are reminded that major barometric falls are a reasonably foreseeable event that should be addressed in the ventilation control plan.”

Barometric trending should include both the effects of diurnal barometric change as well as major barometric events. 

Mine Safety said to assist in determining barometric trend, barometric monitoring should be installed.

“Mine operators are reminded of the effects a falling barometer can have on sealed goaf areas, and hence methane levels in accessible roadways,” it said.

“Mine operators should prepare appropriate trigger action response plans supported by both real-time and predictive services to understand barometric trend and magnitude and so provide adequate time to allow the implementation of mitigating controls to minimise a potential increase in methane gas levels more than 2%.”