A "mega blaze" is believed to have destroyed several homes in the Hawkesbury area north-west of Sydney, where authorities issued an emergency warning.

Properties are believed to have been lost around Mt Tomah and Mt Wilson but there are no reports of injuries at this stage.

The fire was downgraded overnight to watch and act.

NSW Rural Fire Service (RFS) Deputy Commissioner Rob Rogers said there were reports properties were "impacted" after the fire grew "considerably" at its southern end over the afternoon.

Residents in Mt Wilson, Mt Irvine, Mt Tomah, Berambing and Bilpin have been told it is "too late to leave".

"Firefighters are trying to protect as many properties as they can but people in that area can see that fire's behaviour is really tough," Mr Rogers said.

The NSW RFS is battling a blaze burning in bushland at Mt Wilson. ( ABC News )

He said the dry conditions meant fires were burning ferociously even without winds.

"We're not expecting a huge amount of respite," he said.

He said efforts over the next 24 hours would concentrate on strategies to protect properties ahead of the heatwave predicted to move into the state on Wednesday.

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"People should be under no illusions, we're not going to get these fires put out," he said.

"They're going to be burning and they're going to be burning around homes throughout the next week.

"We're going to do as much as we can to protect as many homes as we can, but I think for those residents in those areas and firefighters, it's going to be a really tough week."

Captain Peter Duff from the RFS Terrey Hills Brigade captured footage of a terrifying wall of flames blazing at Mt Tomah.

In the video, posted to Facebook, a firefighter says they are tackling "spot fires galore" and flames 60 to 70 metres high.

A firefighter tackles a fire at a property in Mt Wilson. ( ABC News )

Elsewhere, residents in Bilpin have been warned to look out for embers, which are travelling five to six kilometres ahead of the flames due to windy conditions.

Several roads around the region have been closed including Bells Line of Road, where there are fires on both sides.

The "mega blaze" at Gospers Mountain has been raging for several weeks and has burnt more than 369,000 hectares.

The RFS said there were hundreds of firefighters on the ground, as well as airtankers launching fire retardant to help prevent the fire's spread.

A severe heatwave is expected to hit NSW by the end of the week, according to the Bureau of Meteorology.