NSW Premier Barry O'Farrell and senior police are warning people to abide by total fire bans across the state as Sydney tracks towards its hottest October day on record.
The temperature hit 34 degrees Celsius at Sydney Airport by 12.25pm (AEDT) as several other areas of the city and across NSW climbed above 30C.
Tops of 39C are predicted for Sydney, while western regions can prepare for up to 42C.
Fire and Rescue NSW said it had mobilised almost 250 fire engines and 1000 firefighters after bringing in reserves and recalling off-duty personnel.
Mr O'Farrell said strong wind gusts predicted for later Thursday combined with the hot weather meant out-of-control fires were more likely to occur.
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"Today's a day which, if everybody behaves responsibly ... we will hopefully avoid those consequences," he told reporters.
Fire bugs would be shown no mercy, Sydney general metropolitan region commander Assistant Commissioner Mark Murdoch said.
"If they want to behave in a manner where they come to our notice and they start a fire ... they can expect absolutely no mercy. I mean no mercy," he told a media conference.
By midday, the bushfire situation was stable with the NSW Rural Fire Service not reporting any emergencies.
However, a small fire in a building in Sydney's CBD resulted in the evacuation of about 100 office workers.
Ambulance NSW's Jackie Levett said there had already been 24 cases of people fainting by late Thursday morning - a higher rate than usual and most likely due to the heat.
All tracks and trails in Sydney's national parks have been closed as well as some vantage points for the departure of the International Fleet Review tall ships, the Office of Environment and Heritage says.
