As NSW braces for severe weather, including an expected top of 41 degrees Celsius in Sydney, police are investigating the cause of an overnight bushfire in the Blue Mountains.

NSW Police has reactivated its arson taskforce ahead of expected near record-breaking temperatures and strong gusty winds, which could turn much of the state into a tinderbox.

Sydney's CBD is barely visible through air pollution as the mercury heads towards 40C on Friday. ( AAP: Dean Lewins )

At 11:10am, the temperature at Sydney's Observatory Hill was 32.3C, while at Sydney Airport it was 34.1C.

At Gosford, on the state's central coast, the mercury had reached 36.5C.

A bushfire broke out in the Blue Mountains town of Hazelbrook about 3:00am and had engulfed about an acre of bushland when crews arrived.

They soon brought it under control and police said no property was damaged.

Inspector Peter Balatincz said it was unclear how the fire started, but he called for anyone who witnessed suspicious behaviour in the area overnight to contact police.

Today's forecast maximum of 41C for Sydney is less than one degree short of the November record of 41.8C, set in 1982.

The extreme heat, expected across much of the state, has been caused by a cold front, moving up from the south, meeting a sluggish high pressure system over the Tasman Sea.

It is expected to suck hot, dry air from central Australia into NSW.

A warning for damaging winds was also in place this morning, with strong north-westerly winds coming ahead of the front.

Brief periods of gale force south-westerly winds are also expected, as the front moves over the south and far western inland of the state during the late afternoon and evening.

There could be peak gusts up to 90 kilometres an hour in the Illawarra, Southern Tablelands, Riverina and the Snowy Mountains and the Australian Capital Territory.

Total fire bans in southern NSW

The Bureau of Meteorology (BoM) has also issued extreme fire danger warnings for parts of the Riverina as well as the Shoalhaven and southern ranges.

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The Rural Fire Service (RFS) issued total fire bans for the Illawarra/Shoalhaven, the Southern Ranges, and the eastern, southern and northern Riverina.

The Riverina districts are classified as having an extreme fire danger, while the Illawarra and Shoalhaven are listed as severe.

RFS spokesman Paul Best said firefighters would be on alert during the "really hot, really dry, really windy, dangerous conditions".

He said there was relief in sight, with a southerly change expected to come through later in the afternoon.

"We are expecting a hot day throughout, maximums in the 40s ... with really low humidity and really strong west-north-westerly winds right across NSW, ahead of a strong southerly change that will start moving up the coast from around midday," he said.

People across the state are being urged to make sure they have a fire plan and to know how to act on it.

"People need to be really aware of any fires that break out today, they really need to be across it," Mr Best said.

Yesterday afternoon, farmers across the Riverina were asked to stop harvesting because of the extreme fire risk.

The RFS said similar requests were likely again today.

Sorry, this video has expired Hot chimps cool off at Taronga Zoo ( Rachel Pupazzoni )

Police working to stop firebugs

Detectives from the State Crime Command have reactivated their bushfire arson taskforce, Strike Force Tronto.

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It is the lead investigative body on suspicious or deliberately lit bushfires which lead to death, serious injury or significant property loss.

The Strike Force's Commander Detective Inspector Ian McNab said he wanted the public to contact officers if they had any suspicions that people are deliberately starting fires.

"That's where the role of the community cannot be overstated," he said.

"Information provided to us by members of the public in the past, has helped us identify and suppress firebugs before they start lighting.

"If you have any suspicions at all about someone who you believe could be involved in arson, don't leave it to chance, make sure you contact the police."

NSW Health is urging people to keep hydrated and avoid hot or sugary drinks.

It is also suggesting people limit physical activity and check in on elderly neighbours or relatives to make sure they are coping.