There are fears unpredictable weather conditions could cause a mega-fire in the Blue Mountains if a number of existing large blazes join together.

The front of the State Mine fire near Lithgow already extends across hundreds of kilometres, and this morning authorities deliberately join the blaze with the Mount Victoria fire to the south.

Under what they said yesterday was the "worst possible scenario", it could possibly merge with the Linksview Road fire in Springwood, to the south-east, which destroyed almost 200 homes on Thursday.

Dr Owen Price, a senior research fellow at the Centre for Environmental Risk Management of Bushfires at the University of Wollongong, says there is a risk that a combined fire could produce so much energy it could create its own weather conditions.

A volunteer firefighter puts out a spot fire near the Monkey Creek Cafe in the town of Bell. ( AAP: Paul Miller )

"You can get these conditions, what you call a pyrocumulus, where a fire is producing so much energy it punches up through the troposphere a huge plume of smoke, essentially creating a thunderstorm with lots and lots of energy in it," he said.

"Then it starts to suck in air from all around, so there's more oxygen and it feeds back on itself so the fire behaviour goes really extreme.

"Unfortunately under those conditions when it's creating its own weather you can get things like tornados occurring.

"If two big fires coalesce together they're sort of pooling their energy together, so you can get feedback that makes them even more intense."

Dr Price says fires that serious can become impossible to fight.

But he also says the Blue Mountains fires could join up without necessarily getting much worse.

"It's not a given that if these two fires will meet that that's what they will do, they could just continue the way they're going. But there's a risk," he said.

Fire crews employ high-risk strategies

Firefighters are now employing what they call "high-risk" back-burning strategies to fight the fires, including back-burning, working with bulldozers and working with hand tools to create fire breaks.

"Imagine lighting up 10 to 15 kilometres of countryside along a very windy road in some of the most steepest, scenically beautiful rugged terrain, against a prevailing, hot, dry wind and expecting it to go away from you. It's a challenge," Commissioner Shane Fitzsimmons said this morning.

Sorry, this video has expired Rick McRae from Canberra's Bushfire Cooperative Research Centre explains what a fire tornado is to ABC News 24 in November 2012.

Dr Price says much depends on weather conditions over the next few days, which are difficult to predict.

But he says there are two main possibilities.

"One is that it's going to be dry for days and possibly weeks, similar conditions to what we've had over the last several weeks, in which case [the fires] are just going to keep expanding," he said.

"There's a fair chance that tomorrow evening we might get some fairly heavy rain - 20 or 30 millilitres - which might not be enough to extinguish them but certainly to put the brakes on them for a while. Rain is what will eventually put these fires out.

"When we get a really good fall of rain, something in the order of 60 to 100 millimetres, is hard to predict."

Perfect storm of fire conditions

Scientists at the Bushfire Cooperative Research Centre predicted in September that parts of southern Australia could have above normal bushfire activity this season.

Higher-than-average temperatures over the past year mean grasslands and forests have dried out, providing plenty of fuel to burn once fires start.

"We know that bushfires will occur every year," Bushfire CRC chief executive Gary Morgan said in an email.

Destroyed houses are seen in the aftermath of bushfires at Springwood in the Blue Mountains. ( AAP: James Brickwood )

"When the conditions are right, hot and windy days, with dry vegetation, fires will occur. We all must be vigilant about our local conditions."

In many parts of New South Wales, the high temperatures also mean hazard-reduction burns could not occur at the scale necessary before the fire season began.

Dr Price says recent dry weather and high winds made conditions in the Blue Mountains especially dangerous on Thursday.

"We haven't had any serious rain for two or three months and it's been unusually warm for this time of year," he said.

"Last Thursday, the wind speed was very, very high; 80 kph winds really drive fires and any spark would have set them off."

That combination of high temperatures and winds, he says, is what makes the coming Wednesday so risky.

"Not only do we have the possibility of new fires starting like they did Thursday, but we have all those fires that are already alight, with a 300-kilometre fire front already established," he said.

What turns a spark into a bushfire?

About half of all bushfire ignitions come from lightning, but those deliberately or accidentally lit by humans tend to be closer to populated areas and more likely to damage property and infrastructure.

Sometimes the high winds that contribute to dangerous fires spreading are also responsible for starting them.

The bushfire that started at Linksview Road in Springwood was reportedly sparked after winds knocked over a power line.

Residents were put on high alert last week when the wind changed and the Lithgow fire headed towards Bilpin. ( Audience submitted: Michael Andrews )

"If you get some sort of spark on the ground, if it's sufficiently dry and there's sufficient energy in that spark, it can cause the litter that's lying there, usually grass litter or dead litter, to ignite," Dr Price said.

"That can spread. Some of the factors are to do with the amount of moisture in the dead leaves. If there's low moisture it can spread with a certain intensity. If it's below a certain intensity it will just self-extinguish, but if it's above a certain intensity it will spread.

"With conditions like we had on Thursday, short-term drought plus very dry air, windy air, it didn't take much; the intensity that required it to spread was instantly available."

Bushfires travel at different speeds depending on the terrain and the kind of vegetation growing, but eucalypts are especially adapted to spread fires.

"In eucalypt forests throughout Australia you can get lofted pieces of bark and sometimes leaves that travel a long way," he said.

"Eucalypts evolved in Australia 15 million years ago, under a fire-prone environment. They are perfectly adapted to survive the fire and to spread the fire. We have a fire-prone continent and it's all part of the system."