But new research prompted by the Canberra fires of 2003 is now helping firefighters predict the sudden change and save lives.

For the first time, what was unpredictable and deadly is now much easier to foresee and protect against.

A team from the University of New South Wales in Canberra has studied combinations of terrain and types of fire plus atmospheric conditions to help firefighters spot the conditions where a fire suddenly turns savage. Its system is being used by the the ACT Emergency Services Agency, which is advising firefighters across Australia.

Academic mathematician Jason Sharples was prompted by the 2003 fire in the nation's capital to join the Rural Fire Service - and also to turn his mathematical skills to the study of firestorms.

"After 2003, there was a whole lot of questions asked about why the fires burned so spectacularly," the associate professor said.

He and his team combined analyses of the atmosphere with experience on the ground in a way that hadn't been done before. Many variables were combined to produce a new method to predict which fires were about to turn seriously nasty and become firestorms.

In May last year, it twice predicted the change to a firestorm correctly as bushfires consumed 55,000 hectares of land in central New South Wales around Leadville and Cassilis.

A firestorm is a very different beast from an ordinary bushfire.  It creates fierce winds and moves unpredicatably like an enraged animal. Intense heat can rise up to 15 kilometres into the air and trigger a thunderstorm where embers rain down instead of water, and that means that the fire moves in utterly unpredictable directions, sometimes against the wind. It can roar one way one moment and then reverse the next.

According to Dr Rachel Badlan, who is also in the UNSW Canberra research team, one of the key factors is the shape of the fire on the ground. If it is made up of many thin strips of fire-fronts - even though the total area may be large - there is less chance of the fire escalating to a firestorm than if the fronts join together with the interior igniting and becoming a whole block of fire.

That generates a concentrated area of intense heat, which rises and disrupts the atmosphere. Rugged terrain makes the fire worse and more likely.

"When a fire becomes a square or a circle - when it becomes a big blobby mass - that seems to be a precursor," she said.

This matters because fire crews have to make big decisions when fighting fires. Dr Badlan said the ability to predict a firestorm meant the RFS could decide to evacuate an area much earlier than previously.

"You can't suppress firestorms because of the erratic winds," she said. "They can go in any direction. There's no point in deploying fire crews. The best thing is early evacuation."

She said firestorms were becoming more common. "We've had 50 plus since 2001 and they are increasing due to climate change."