Fierce bushfires continue to encroach on homes in NSW, as thick smoke blankets Sydney for the fourth day in a row.

Key points: The Three Mile fire in the Hawkesbury has destroyed at least one property

The Three Mile fire in the Hawkesbury has destroyed at least one property Three fires had been burning at emergency level in the Lower Hunter but are now back to advice

Three fires had been burning at emergency level in the Lower Hunter but are now back to advice Thick smoke continues to cover Sydney and surrounds

The NSW Rural Fire Service (RFS) said there were 137 fires burning across the state as of 6:15pm.

More than 2,500 firefighters are battling the blazes ahead of worsening conditions expected on Thursday.

There is severe fire danger predicted for seven areas of the state on Thursday: Greater Sydney, Greater Hunter, Central Ranges, Southern Ranges, Illawarra/Shoalhaven, Monaro Alpine and Far South Coast.

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At least one property was engulfed on Wednesday in the Three Mile fire in the Wisemans Ferry area on Sydney's northern outskirts, with other buildings under threat.

It is believed the property is part of a Buddhist monastery complex — a water bombing helicopter could be seen trying to save the meditation hall at the forest monastery, which trans monks and lay preachers at Ten Mile Hollow.

Another three fires — one at Gospers Mountain and others at nearby at Paterson and the Howes Valley in the Lower Hunter — were burning at emergency level.

All three have since been downgraded back down to advice.

Smoke from the Three Mile blaze on the north side of the Hawkesbury, which is burning across more than 5,800 hectares, had been moving across into other regions including Sydney City.

It is currently rated watch and act by the NSW Rural Fire Service.

Winds are causing significant smoke from Mangrove Mountain and further east towards the Central Coast.

People in the areas of Mangrove Creek, Greengrove, Lower Mangrove, Upper Mangrove, Central Macdonald, Lower Macdonald have been told to activate their bushfire survival plan.

The RFS warned that conditions will worsen with winds and temperatures expected to continue to increase until Friday.

Elsewhere, the bushfire burning for weeks at Gospers Mountain jumped back to emergency level on Wednesday afternoon, with properties under threat in the Colo Heights area.

A new emergency level fire also developed at Paterson in the Lower Hunter region, where residents in the area of Red Hill and Duns Creek had been told it is too late to leave.

In the nearby Howes Valley, south-east of Singleton, the Paddock Run fire has burnt more than 5,800 hectares and residents in Howes Valley and Reedy Creek had been told to seek shelter.

A ferocious blaze continues down the South Coast at Currowan, which has now razed through almost 50,000 hectares of land.

Firefighters at work on the Currowan fire between Batemans Bay and Ulladulla earlier this week. ( AAP: Dean Lewins )

There are total fire bans in place for nine areas from midnight: the Greater Sydney region, Greater Hunter, North Western, Northern Slopes, Central Ranges, Illawarra/Shoalhaven, Southern Ranges, Monaro Alpine and Far South Coast.

Ten schools across the state will be closed on Thursday because of the bushfire risk, the Department of Education said.

Smoke risk continues

Meanwhile, Sydney and surrounds have been blanketed again in thick, hazardous smoke in what social media users are calling the "new norm".

People were seen wearing face masks in Sydney on Wednesday. ( AAP: Bianca De Marchi )

One Twitter used said: "We are fearing this is going to the norm during all the summer."

Another said: "Bushfire smoke is gradually becoming the new norm for Sydney. Which is scary and heartbreaking."

The NSW Bureau of Meteorology tagged city the "brown town" due to the smoke haze, which it said will continue into Thursday.