The fire danger conditions experienced in Sydney over the weekend were at their worst since the early 1950s, according to one of Australia's top bushfire experts.
Firefighters are still trying to bring a large bushfire in Sydney's south-west under control after it was believed to have been deliberately lit at Casula on Saturday.
Dozens of people were ordered to evacuate from suburbs, including Menai and Barden Ridge, where the blaze came perilously close to homes.
Hundreds of firefighters were brought in from a wide area of Sydney to help safeguard properties.
The Rural Fire Service downgraded the bushfire to Advice level after gaining the upper hand on Monday, but said the blaze remained dangerous and unpredictable.
Ross Bradstock, the director of the Centre for Environmental Risk Management of Bushfires at the University of Wollongong, examined weather data recorded at Sydney Airport since the early 1950s.
Professor Bradstock said the conditions experienced in Sydney on Friday and Saturday resulted in a Fire Danger Index of 60, when the historical average for this time of year is 40.
"What we've seen over the weekend is weather conditions, in terms of fire danger, that are unprecedented in the historical record, effectively the living memory of most people who live in Sydney," he said.
He said Sydney could expect a day of about 60 on the Fire Danger Index around once a year, typically between October and December.
"So to have a day where it's about 60 in mid-autumn is quite crazy really, to put it bluntly.
"It's worth saying also in the past the maximum fire danger is about 40, which is classified as very high. So we've now seen days that are classified as severe, but there's a big difference between 40 and 60.
"And we know from looking at the history of property loss from bushfires in the Sydney region, that when you go from 40 to 60, you essentially jump up into a set of conditions where you are going to have property loss — close to a 100 per cent chance."
Professor Bradstock said only the skills of the firefighters prevented that becoming a reality.
It's becoming a 'global' fire season
Authorities are facing the possibility of having to plan for a much longer fire season in Australia of about six or seven months, which would require greater resources, according to Professor Bradstock.
"There has been some analysis generally in trends of fire danger across Australia, and there has been an upward trend since the 1970s, according to annual records," he said.
He said the upward trend was most pronounced over south-east Australia, the most heavily populated part of the country.
"I think the bottom line would be that it will cost more money," he said.
"You'll need more equipment, or at least you will have to maintain people and equipment in a high state of preparedness for a longer period of time and that will cost more money."
He said Australia would need to rethink its arrangements to share waterbombing aircraft with authorities in the northern hemisphere, in the wake of unprecedented fire seasons in North America and Europe last year.
"So we're almost looking at a situation where the fire season globally is never ending, and if we're doing equipment deals with countries like the USA, it means the turnaround window may start to disappear," he said.
Fire authorities in NSW are talking to the Federal Government about additional funding for vehicles and aircraft.
RFS Commissioner Shane Fitzsimmons said the state's fire season traditionally finished in March, but this year it had been a longer than usual.
Two large air tankers that were normally used had already been sent back to the United States for its fire season.
The commissioner said the strategies were being reviewed.
"We are working with the Commonwealth about funding strategies that might see our capacity grow to extend the presence of those high-capacity assets, particularly the large air tankers that have the ability to get far and wide in short distances," he said.ABC