A ''DOME of heat'' is expected to push temperatures in NSW above 40 degrees, creating ''catastrophic'' fire conditions.
The Premier, Barry O'Farrell, made an emphatic appeal to all residents to be fully prepared for the worst as temperatures soared on Tuesday.
''[It] is not just going to be in the 40s. It will perhaps be the worst fire danger the state has ever faced,'' he said.
A total statewide fire ban was imposed last night. The Shoalhaven and Illawarra area and the southern ranges are the most at risk, categorised as having catastrophic conditions.
Most of NSW - including Sydney, the ACT and the central west - is rated as ''extreme'', while in the far west and on the far south coast there are warnings of ''very high'' fire danger.
Firefighters from Fire & Rescue NSW will assist the Rural Fire Service. National Parks and camping grounds will be closed and nursing homes and vulnerable people in badly affected areas will be evacuated.
The Fire Commissioner, Greg Mullins, said: ''We are expecting extremely dangerous bushfire conditions, the likes of which haven't been seen in NSW for many years. It is 19 years to the day that we saw more than 100 homes destroyed in southern and northern Sydney.''
Mr O'Farrell urged campers and holidaymakers to head to safer ground as early as possible. Thousands of fire service personnel were on standby and canvassing areas at risk.
''Rethink your need to be in the bush, have a bushfire plan, be prepared but most importantly make sure you are well away from harm's way,'' he said.
''Don't just think, [it] 'is another bushfire danger day, tomorrow is another summer's day'.
''[It] is going to be the worst fire danger day in parts of this state we've ever experienced.''
In some towns in the far west the temperature is predicted to hit 45 degrees.
In Sydney the mercury is forecast to reach 43 degrees. The hottest day on record for Sydney is 45.3, on January 14, 1939, followed by 44.2 on New Year's Day, 2006.
Experts have described a huge "dome of heat" over Australia that has broken national temperature records and set the scene for dangerous fire conditions for days across large parts of the country.
Australia is likely to have posted a record average maximum on Monday, beating the previous high of 40.17 degrees on December 21, 1976, said Karl Braganza, manager of climate monitoring at the weather bureau.
He said that while such records were ''hard to break" and the bureau was yet to complete its analysis of the numbers, the previous high would probably be exceeded on Tuesday.
"It's likely to just beat it," said Dr Braganza, adding that the mercury was continuing to climb in some areas of the nation.
The heatwave continues to envelop a huge region, from northern Victoria and inland NSW, to Birdsville in central west Queensland, to north-western Western Australia and down to the Nullarbor coast.
"It's just an extensive dome of heat over the continent," Dr Braganza said.
The NSW Rural Fire Service Commissioner, Shane Fitzsimmons, said winds of up to 80km/h would make things more difficult for emergency services.
''It's not just the heat and winds but because there is so much dry fuel and little humidity, it's going to be a shocking day,'' he said.
He said there were more than 90 fires on Monday, 20 of which remained uncontained.
On Monday residents of Oura in southern NSW were warned not to leave their village but to take shelter if a nearby out-of-control grassfire threatened homes. A fire broke out between Gunning and the Belmont Forest near Goulburn and has raced through more than 25 hectares.
Mr Fitzsimmons encouraged campers and those in potentially fire-affected areas to remain vigilant but act accordingly. ''We're expecting awful conditions tomorrow. Monitor your local conditions. Listen to radio. Look at the TV. Access social media tools. Stay tuned to what's happening,'' he said. ''Complacency costs lives,'' he said. ''Be alert, be aware.''
Alongside firefighters, 60 to 70 aircraft will be on standby.