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 150 fires continued to burn across New South Wales state on Sunday, with 39 more in neighboring Victoria, after conditions turned catastrophic a day earlier when temperatures surged past 100 degrees Fahrenheit and blazes were fanned by strong, unpredictable winds. Wildfires have blackened more than 20,000 square miles so far.