An Anglo American spokeswoman said while underground access at the mine remained restricted, the company had not been able to confirm the scale of the June ignition event.

 "Since ceasing longwall mining activities on May 6, we have been closely monitoring the resulting levels of various gases to ensure the ongoing safety and integrity of the mine," she said.

"The suspension of longwall mining elevates the risk of oxidation of the coal in the longwall goaf environment. During normal longwall mining operations this risk is very low.

"The steps we are taking to address the coal oxidisation and heating risk at Grosvenor are tracking as expected, and we believe that the oxidation in the goaf is currently under control."

The spokeswoman said the company made the decision to withdraw personnel from the underground environment last month in line with its risk management processes and had been working with its expert technical advisors to assess and respond to the situation since then.