A 60-kilometre fire front is ravaging areas north of Sydney, as more than 100 blazes take hold amid deteriorating weather conditions.

The Bureau of Meteorology (BOM) warned "some fires were too big to put out" and the level of smoke was such that it was showing up on its radar as rain.

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NSW Rural Fire Service (RFS) deputy commissioner Rob Rogers said there were "probably more than eight fires" in the area north of Sydney that had combined.

"They are all in a 60-kilometre stretch that runs from the Colo area in the Hawkesbury, north of Sydney, all the way up to near Singleton," he said.

"There is just fire that whole way."

A thick blanket of smoke descended on Sydney again on Friday afternoon, with the RFS warning it would last into the night and people should limit their exposure.

There were seven emergency warning-level fires blazing across the state at 7:00pm on Friday, but they had all been downgraded to watch and act or lower by 11:15pm.

An eerie glow descended on Sydney Harbour late Friday afternoon. ( ABC News )

More than 680 homes have been destroyed by bushfires in NSW this season, the RFS confirmed.

Almost 250 houses have been damaged, while more than 2,000 outbuildings have been destroyed or damaged.

A total fire ban is in place for seven regions on Saturday including Greater Sydney.

Rod Meggs puts out spot fires on his property in Kulnura, on the Central Coast. ( AAP: Dan Himbrechts )

The updated totals came as the blazes burning north-west of Sydney joined to create what has been dubbed a "mega blaze" on Friday morning.

The Gospers Mountain fire in the Hawkesbury region merged with two fires in the Lower Hunter — the Little L Complex blaze and the Paddock Run fire near Singleton.

The three fires have burned through more than 300,000 hectares and destroyed about 10 homes.

Thick smoke blankets Kulnura as fire approaches Mangrove Mountain north of Sydney. ( AAP: Dan Himbrechts )

NSW Premier Gladys Berejiklian has urged residents to remain vigilant.

"We have seen the fire is coming to very close proximity to major population centres, whether it is on the South Coast, Central Coast, or even Greater Western Sydney," Ms Berejiklian said from the RFS control room in Sydney.

"We know there have been some property losses, the extent of which won't be known until the fire conditions are safe for people to go in and have a look, but we also appreciate that many families and many people have been asked to leave their homes and we can appreciate what a scary time this is for many people."

NSW RFS crews work to protect a property in Kulnura from the Three Mile fire. ( AAP: Dan Himbrechts )

The RFS said on Friday afternoon there were more than 100 fires burning around the state.

More than 2,800 firefighters are at work in conditions the RFS described as "challenging" due to high temperatures and strong winds.

Total fire bans are in place for 10 regions in NSW, including Greater Sydney, where the fire danger rating is "severe".

The sign at Laguna Public School tells the story on Friday. ( ABC News: Ben Millington )

Warm and dry winds are expected to continue to elevate fire dangers across north and east parts of NSW.

At one stage last month, emergency warnings were issued for 17 separate blazes on one day.

Fire crews confronted by tall flames in Kulnura. ( AAP: Dan Himbrechts )

On Thursday, the RFS issued emergency warnings for seven fires, including two threatening areas on Sydney's doorstep.

Deputy commissioner Rogers said crews would assess the damage from those blazes on Friday and over the weekend.

"I think that firefighters did some incredible saves last night on properties and I think some of the vision has shown that," he said.

"We've got to work out how many homes we have lost."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 45 seconds 45 s Crews retreat as wall of flames engulf bush near Orangeville

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On Thursday, firefighters filmed the moment a wall of flames began surrounding them at Orangeville, just 70km from Sydney's CBD.

Mr Rogers said those conditions were "representative" of other areas in NSW.

"The fires in the north, they were dealing with these sort of horrendous conditions, down in the Shoalhaven and Sydney," he said.

"It's reflective of how dry the landscape is.

"The drought makes the fuel so quick to burn and it burns so volatile when it burns."

Firefighting and incident management specialists from Canada were briefed in Sydney on Friday morning.

They will be deployed throughout NSW on Saturday to add their expertise to the battle to control bushfires burning across the state.