Residents near a bushfire burning through large swathes of the Wollemi National Park, near Sydney, are being warned to "leave before it is too late".

The blaze at Gospers Mountain, about 40 kilometres north of Richmond, in Sydney's north-west, is burning out of control and is currently rated as watch and act by the NSW Rural Fire Service (RFS).

Key points: Fire crews are battling the 85,000-hectare Gospers Mountain blaze on several fronts

Fire crews are battling the 85,000-hectare Gospers Mountain blaze on several fronts Smoke from the fire is blowing over the Central Coast and Sydney

Smoke from the fire is blowing over the Central Coast and Sydney More than 250 homes in northern NSW have been destroyed in the past seven days

Several homes and a sawmill are under threat in Colo Heights, on the south-eastern side of the fire.

In addition to Gospers Mountain, emergency warnings were issued for blazes at Bora Ridge, Ebor and Willi Willi, on the state's Mid-North Coast. All four emergencies have been downgraded to watch and act status.

Total fire bans have come into force in four regions in northern New South Wales. The bans will be in place on the far north coast, the northern slopes, the New England and north-western fire regions.

Police have arrested a 51-year-old man for allegedly lighting the Ebor fire. He is being questioned at Armidale Police Station.

The RFS said the Gospers Mountain fire was "fast moving" and spreading quickly amid hot and windy conditions.

Crews are fighting the 85,000-hectare fire on several fronts.

On one side it is moving south towards Colo Heights where people are being advised to move in the direction of Wilberforce.

It is also spreading north towards Yengo Drive, where people are being told to move in the direction of Milbrodale.

Luke Ether from the RFS Hawkesbury brigade said firefighters were setting up to provide property protection around Colo Heights.

"The fire front is coming towards us but right now it's slowly starting to die down so hopefully we're all good," he said.

Mr Ether said with the wind behind the fire, it was possible the blaze could intensify.

"[These fires] are pretty dangerous to fight but that's what we're here for."

Danny Campos has decided to leave Colo Heights. ( ABC News )

Resident Danny Campos was taking the RFS advice and leaving towards Windsor, with little more than some medication and sleeping bags.

"I'm going to town, leaving now — it's scary," he said.

NSW RFS Commissioner Shane Fitzsimmons said conditions were not helping firefighters.

"The winds particularly are stirring up the fire behaviour and causing the fire to start becoming more erratic and spreading," he said.

"These four that have triggered into emergency warning are because they are either directly or imminently about to impact on more property or more people and communities."

Mr Fitzsimmons said temperatures over the weekend would be "challenging", however the real threat would come mid-next week with another burst of hot and windy weather.

"There's just so much fire on the ground there's so much potential for these fires to continue to spread," he said.

It has been a dire week for NSW firefighters — in the state's north, more than 250 homes have been destroyed by blazes in the past seven days.

The RFS has been assessing the damage from a series of out-of-control blazes since last Friday and northern NSW was affected particularly badly.

A total of 259 homes were razed in that area, while 87 homes were damaged and almost 500 outbuildings were destroyed.

Four people have also been killed.

Last Friday, a record 17 emergency warnings were issued for fires burning in NSW.