The fire at the Blakefield South underground mine last week appears to have been caused by methane gas, according to mining union officials.
They say the blaze, which is still burning, was the result of an explosion or ignition of methane, and they will continue to monitor the levels inside the mine.
District chief inspector Keith Shaw said levels of combustible gases are still coming from the fire are and it could be 10 days before they have the blaze extinguished.
"It was an ignition of methane but until we get in there to have a closer look the rest of it is conjecture,” he said.
"One bloke said he saw flames behind the longwall face at the tailgate side of the longwall," Mr Shaw said.
Construction, Forestry Mining and Energy Union district vice president, Greg Sharp said there were about 50 men underground when the fire started, and were “incredibly lucky to get out as they did.”
Xstrata Coal says it could be months until the $350 million mine operation is reopened, and workers have been redeployed in the meantime.
James Rickards, spokesperson for Xstrata Coal said there will be a meeting on Friday between union officials and Xstrata Blakefield workers.
"The company has kept the permanent and casual workforce on pay but things are not looking so good for the contract workers," he said.
The Blakefield South operation employs about 550 people, with 220 union members, about 80 staff or mine deputies and the rest contactors, according to Rickards.
He says the total cost of the fire is still unknown.
"With the mine out of action there would be costs in the millions to us," he said.
"But we’re yet to effectively assess that and once we get underground then we’ll be able to determine what sort of damage has been done."
Rumours that the fire had started from a lighting strike hitting a gas vent have been discounted, according to Union district president Peter Jordan.
Xstrata coal said they will continue to monitor gas levels in the mine and no-one will be sent back underground until they are certain it is safe.
"Our teams know how to do that, they’ve been doing it well," Rickards said.
"We’ve got an excellent drain gas system in place.
"We manage our gas very efficiently, full credit to our team that they have the training and expertise to respond efficiently to an emergency situation, no one was injured or there was no event that caused any level of concern to our people."