A combination of extreme temperatures (38 to 44-ish Celcius), low humidity (means bloody bone or tinder dry), strong gusty winds and highly flammable material in Australia’s oil-rich dry eucalyptus bushland, make for frightening spread of bushfires. Some pyromaniacs add to the danger – there seems to be a number of such sick people.
In January 2003, the capital of Australia was nearly wiped out. The terrified population was standing by for mass evacuation. T’was only a wind change that saved the entire city, though a number of unfortunate suburbs were razed to the ground. I can tell you that it's not funny being caught by the fire. With the gusty wind conditions, the bushfires move faster than one can run or even cycle.
One of the most frightening phenomenon of a raging out-of-control bushfire is the vortex it creates - it sucks in hugh volumes of air and what else, burns then as well as chucks them up into the air before gravity resumes its dominant status. In the 2003 Canberra fires, there were reports that burning branches as thick as a man's thigh was flung several kilometres away from where they originated. So being some distance away from a raging fire won't necessarily be safe.
BTW, the temp in Sydney today reached