A total fire ban has been declared for 15 areas in NSW ahead of hot and gusty weather conditions.
Sydney, the Blue Mountains, Central Coast, Illawarra, Shoalhaven and Greater Hunter areas are forecast to face extreme fire danger on Thursday, the NSW Rural Fire Service (RFS) says.
Total fire bans will be in place for those areas and 12 others, including northern, western, and southern parts of the state.
NSW RFS Commissioner Shane Fitzsimmons said there was the potential for some locations to reach "catastrophic" fire danger - the highest on the bushfire danger scale - if the weather deteriorated.
"These are dangerous conditions, and it's critical that you understand the risk and make your final preparations now - do not leave it until the last minute," he said.
"Under these conditions, any fire that starts and takes hold will spread quickly and may threaten homes and lives without warning."
The Bureau of Meteorology is forecasting a maximum 37 degrees in Sydney on Thursday with north-westerly winds of up to 50km/h.
The RFS recommends residents living in or near bushland to ensure everyone in their family knows how to act if there is a fire.
Emergency services are also appealing to the community to keep an eye out for and report firebug activity.
Over last year's bushfire danger period - between October and March - 87 people, including 55 juveniles, were the subject of legal action for 117 bushfire-related offences.
Police Assistant Commissioner Alan Clarke said anyone caught deliberately lighting bushfires could face penalties of up to 25 years' imprisonment.
Strike Force Tronto is in place to investigate suspicious or arson-involved bushfires that lead to death, serious injury or significant property loss.
Bureau of Meteorology forecaster Olenka Rudewych says Sydney and the Hunter areas are facing temperatures between 37 and 39 degrees on Thursday.
But it will be worse in the far-northwest where the temperature is tipped to reach between 41 and 42 degrees.
Ms Rudewych confirmed the temperatures were unseasonal.
"The average temperature for October (in Sydney) is 22 degrees so yes, it's really warm for this time of year," she told AAP.
North-westerly winds of 60-70km/h and gusts of up to 100km/h are forecasted for southern parts of NSW.
The southern ranges could see winds of up to 120km/h.
Ms Rudewych said temperatures would cool and winds would ease for Friday.