The union says the gas level reached a near-explosive reading after ventilation fans cut out last Tuesday and the risk was exacerbated by workers not being evacuated immediately.

Construction, Forestry, Mining and Energy Union (CFMEU) district vice-president Bob Timbs told the Mercury fans within a shaft failed “due to a power trip” about 12.30pm.

Mr Timbs said, as a result, there was a build-up of methane in part of the mine. 

“There was a peak reading at 1.53pm [on Tuesday] of 4.73 per cent methane, so that was very close to the explosive level. Explosive levels are between five and 14 per cent,” he said. 

Mr Timbs said all mine operations should have ceased and workers subsequently evacuated “so that the possibility of an ignition source is fairly well depleted”.  “It didn’t happen, they didn’t evacuate the mine,” he said.

The incident sparked an investigation by the NSW Resources Regulator, who issued South32 – Illawarra Metallurgical Coal’s owner and mine operator – a prohibition notice on Wednesday.

The notice halted production activities at the Appin Colliery and “required the immediate withdrawal of workers” until a “full review of its controls managing the risk of methane gas and ventilation” was completed.

A Department of Industry spokeswoman confirmed the prohibition notice “followed an incident at the mine where methane gas levels exceeded legislated limits”.

“The Resources Regulator has since revised the original prohibition notice to allow a limited number of staff to go underground to perform essential duties and undertake limited coal extraction to mitigate against the development of further safety issues arising from localised strata deterioration,” she said.  

“These arrangements are subject to strict conditions relating to the monitoring of methane gas levels and the immediate withdrawal of workers in the event of a fan failure.”

The notice remains in place until the mine’s controls have been reviewed and the regulator “is satisfied that methane gas risks are being appropriately managed”. 

Mr Timbs said the union would be “closely scrutinising” the company.

In a statement, Illawarra Coal said “operations in some areas of Appin Mine are temporarily suspended to enable investigations into an operational matter”.  

“The company is confident of a timely return to normal operations. Production guidance remains unchanged.”