SYDNEY—Wildfires ravaging much of southeastern Australia burned so hot that they created their own thunderstorms and lightning—similar to conditions during a volcanic eruption or atomic bomb blast.

Firefighter Michael Brearley was leading the defense of Wingello, a small town around 100 miles south of Sydney, when a huge cloud began to form in the distance on Saturday. He knew it was bad news: The area had barely seen a drop of rain for weeks.

Instead it was fire that rained down from the formation of pyrocumulonimbus clouds—created by intense heat driving air rapidly upward in the smoke plume from a wildfire, drawing in moisture and resulting in thunderstorms. Scientists say they are only beginning to understand the weather phenomenon, making its behavior hard to predict.

“When the southerly wind change hit, the cloud collapsed and it just threw fire into the forest immediately south of the town,” Mr. Brearley said. “We screamed for more resources. The fire was on us.”

Around 130 fires continued to burn across New South Wales state on Monday, with 31 more in neighboring Victoria, after conditions turned catastrophic on Saturday when temperatures surged past 100 degrees Fahrenheit and blazes were fanned by strong, unpredictable winds. Wildfires have blackened more than 22,000 square miles so far.