The New South Wales Rural Fire Service says the massive blaze that swept through southern Lake Macquarie last week was sparked by an underground fire, which was virtually impossible to put out.
Almost three-thousand hectares of scrub and bushland were burnt out in the firestorm.
At least three properties were destroyed, and a man died defending his home at Lake Munmorah on the Central Coast.
RFS spokesman Stuart O’Keefe says coal tailings from local mining operations in the region can cause underground fires.
He says hotspots around Rutleys Road at Doyalson were picked up several weeks before but could not be extinguished.
“Although there’s nothing overly obvious we put the cameras back on them again and they’re indicating these high temperatures. Same again, we put the water on them,” he said.
“We think that the coal seam or even peat soils or some vegetative matter that’s well below the surface is ignited by some heat.”
Mr O’Keefe says fire investigators are still recording temperatures of up to 400 degrees around some hotspots.
At Catherine Hill Bay, where the historic Wallarah House was destroyed in the blaze, efforts are still in place to control a large smouldering coal pile.
Stuart O’Keefe says it is estimated to weigh about 200 tonnes.
“They’ve decided to put a coating of clay over the top of this fire,” he said.
“Hopefully starve it of oxygen and at least contain it and not allow it to come out of that pile.”
Mr O’Keefe has dismissed some public criticism of the Rural Fire Service’s back burning operations.
“This is just one of the reasons why we can’t burn in some areas because we don’t want to start these underground fires,” he said.
“So there are some restrictions that apply to these areas.”