The maps, compiled by Rachael Nolan from the University of Technology and University of Wollongong researchers, show forest fuel moisture levels are continuing to decline, elevating the risk of major fires without significant rain.

"It's quite incredible," Ross Bradstock, director of UoW's Centre for Environmental Risk Management of Bushfires, said, noting that even typically rainforest-type areas are showing a rapid drying out.

"You're kicking up dust when you'd usually be picking off leeches."

While winter was exceptionally dry, conditions are turned even more drought-like in September.

Just 0.2 millimetres has been picked up in Sydney's rain gauges so far this month.

With barely a shower predicted for the last week of September, the city could nudge out 1882 - with its 2.1 millimetres - as Sydney's driest September on the Bureau of Meteorology's 159 years of records.

As Fairfax Media reported five weeks ago, vegetation in the Blue Mountains, Wollemi and the state's south coast were then approaching or exceeding critical moisture levels associated with all major blazes in the state since 2000.

"We've gone downhill since," Professor Bradstock said, "It's very clear we are now at the [fuel moisture] levels we were at mid-October 2013."

(See chart below showing how current conditions compare with 2013.)

Fires back then destroyed 200 homes near Winmalee and elsewhere in the Blue Mountains. While the burnt areas are excluded from the fuel moisture maps, other regions such as around Katoomba are showing conditions at least as dry this year.

Saturday's temperatures across NSW were expected to challenge records including around Sydney, with Penrith and Blacktown forecast to hit 37 degrees. Total fire bans are in place from the Hunter region to the Victorian border and a range of inland districts.

Average statewide temperatures may top the current September record of 34.2 degrees set in 2003 and NSW may have its first September 40-degree recording at any location, the bureau says.

The moisture maps, which have been shared with fire authorities, show a stark drop in moisture levels of live plants during this year, as vegetation struggles to draw water from the soil.

"We've basically had a re-run of 1968," Professor Bradstock said, referring to a season when Sydney, the Illawarra and Blue Mountains suffered big bushfires.

(See how moisture levels from living plants have changed this year in the Sydney region compared with 2013.)

Ben Shepherd, a spokesman for the NSW Rural Fire Service, said that this fire season had correlations with some of the bad years for fires, such as 2001-02.

"It's very dry and there's no real rain on the forecast," Inspector Shepherd said.

Many areas will have mid- to high-30s on Saturday, with strong, gusty winds and humidity levels below 10 per cent.

After this weekend's burst of heat, more warmth is expected from the middle of next week.

"It will just further bake the landscape," he said.

Professor Bradstock stressed the low fuel moisture levels were merely the pre-conditions for fires, and ignition points were needed for blazes to get going.

Unfortunately, without major rain events, the cycle of regular spikes in fire danger is likely to be a feature for weeks to come. 

Hot, dry winds from the interior are typically drawn into south-eastern states ahead of cold fronts during this spring pattern. Temperatures rise, and winds strengthen before they swing to the south, creating dangerous fire conditions - as will be seen this weekend.

"You've got the pre-conditions" for big blazes, Professor Bradstock said. "It's going to be very, very nasty."

The dry conditions are not restricted to coastal NSW.

As the chart below shows, parts of Victoria are also relatively dry notwithstanding the wetter, cooler winter and early spring for many areas.

East Gippsland and the Otway Ranges, which take in the popular Great Ocean Road to Melbourne's south-west, are among the areas of concern, Professor Bradstock said.

Sydney Morning Herald.