Crews are fighting to contain a spate of fires burning across NSW and threatening homes on Sydney's western fringe and the Queensland border.
Rural Fire Service spokesman Greg Allan said high temperatures, low humidity and strong winds were the perfect storm for dangerous fire conditions.
More than 1000 firefighters are battling 73 fires as the state swelters in a spring heatwave.
A total fire ban is in force for the Greater Sydney, Hunter, Upper and Central West regions with another to take effect in the North West from Thursday due to the extreme danger caused by the record-breaking September heat.
A total of thirty blazes were yet to be contained as of 6pm on Wednesday, the Rural Fire Service said.
Hot, dry and windy conditions were felt across most parts of the state with west-northwesterly winds battering Sydney throughout Wednesday.
Residents near Wedderburn, five kilometres from Campbelltown in the city's southwest, have been told to prepare for s fire burning in nearby bushland.
Crews also struggled to contain a fire near Wallacia on the city's western fringe while homes near Kanahooka, southwest of Wollongong, were under threat.
An out-of-control grass fire close to the Queensland border saw more than five hectares burn near the regional town of Tenterfield.
Firefighters were able to gain the upper hand and slow the spread of the fires with the threat to properties easing by 6pm.
A cool southerly change moving up the south coast and across NSW has begun to take effect, with temperatures expected to drop to the low 20s for Thursday and into the weekend.
The change will provide relief from the unseasonably hot and dry conditions felt across southern Australia, with temperatures peaking between 8C and 16C above average across much of South Australia, NSW and Victoria over the past few days.
Temperatures in Sydney soared to 33.9C at Observatory Hill on Wednesday, just short of the September record of 34.6C set in 1965.
Penrith in the city's outer west hit a maximum of 35.2C at 2.20pm.
Premier Chris Minns voiced concerns over the soaring temperatures given that it is still early spring.
"It's September and we're already experiencing four days in a row of temperatures above 30 degrees with high winds," he said.
"We are concerned about a horror summer."
NSW is behind on hazard reduction burns, with heavy rain and floods over the past few years preventing the usual hazard reduction burns.
"In the last year we've only met 24 per cent of our quota for hazard reduction burning," Mr Allan told AAP, adding that increased rain has led to a lot of grass growth across the state.
"That's all dried off and ready to burn."
The Bureau of Meteorology on Tuesday declared an El Nino event, warning of hotter and drier conditions that could lead to severe heatwaves, droughts and bushfires, particularly in the east.
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