An Anglo American spokeswoman said it had established an exclusion zone around the entry of the mine.

"No crews will return underground until the underground environment has been proven to be stable,'" she said.

"These crews were undertaking essential work to ensure the ongoing integrity and safety of the mine. All personnel on site have been provided with updated information on the mine conditions and the response steps underway."

Anglo American's Metallurgical Coal business CEO Tyler Mitchelson said the company was still investigating the cause of the methane ignition at Grosvenor last month, and in the meantime its team was monitoring elevated risks due to the cessation of the longwall for the past month.

 "Consistent with our risk management protocols, we made the early decision to fully restrict access to the mine when levels of certain gasses started to rise," he said.

"Over the past couple of days, we commenced a range of measures to stabilise the area.

"Once gas levels are proven to be stable, work will continue to ensure the ongoing safety and integrity of the mine.

 "In the meantime, we will continue to keep our workforce closely updated."

Construction, Forestry, Mining, and Energy Union district president Stephen Smyth said workers at all three Anglo American's underground coal mines in central Queensland were concerned about safety, especially regarding the management of dangerous gases, in the wake of last month's explosion at Grosvenor.

"At Grasstree mine near Middlemount, workers downed tools on Saturday night after protocols were breached restarting a ventilation fan after a power outage, resulting in excessive gas entering the mine and risking ignition," he said.

"At Moranbah North mine, workers have been protesting outside the mine over management's refusal to meet workplace delegates to discuss safety issues including gas management.

"And at the site of the explosion at Grosvenor mine, there are serious concerns that conditions underground have not been properly managed in the wake of the explosion, with continued elevated gas levels and fears of a potential spontaneous combustion event in the disused ‘goaf' area of the mine."

Smyth said gas management was the most critical safety issue in underground mines and safety protocols should be stepped up in the wake of the May explosion.

"It is time for Anglo American to override local management at its Queensland underground coal mines and insist that safety is put ahead of production," he said.

"It is beyond belief that just weeks after five men were nearly killed at Grosvenor, we are hearing reports of corners being cut and management refusing to talk to workers' representatives about safety concerns.

"It reflects poorly on the safety culture at Anglo mines.

"At Grasstree, it's likely the relevant supervisors felt pressure to get the fans working quickly to avoid having to evacuate workers and pause production, but their actions resulted in unacceptable and dangerous shortcuts being taken.

"Site union delegates did the right thing by stopping work due to unsafe conditions as is their right under mine safety laws.

"Unfortunately, with the high rates of casualisation across the industry, not every worker feels confident to stop work when conditions are unsafe."

The Anglo American spokeswoman said: "We strongly reject Mr Smyth's version of events and conclusions.

 "Absolutely nothing comes before safety at Anglo American, and we fully investigate every safety-related incident to ensure that the learnings are captured. We are keeping our workforce updated and we encourage everyone to speak up with any safety concerns or improvement opportunities."

The spokeswoman said with the cessation of longwall mining activity after the methane ignition incident on May 6, Anglo American had been closely monitoring various gas levels to ensure the ongoing safety and integrity of the mine.

The suspension of longwall mining over time elevates the risk of spontaneous combustion, due to the oxidation of the coal in the longwall goaf environment.

During normal longwall mining operations this risk is very low.

"Over the past few days we have seen levels of carbon monoxide and other gases start to rise, such that we enacted our Trigger Action Response Plan for spontaneous combustion risk," the spokeswoman said.

"The current situation is that we are undertaking a range of measures to address spontaneous combustion risk, including the addition of inert gases and changes to the ventilation network.

"Our investigation into the methane ignition incident is still underway and we have committed that we will not recommence mining until it is safe to do so."