Residents near a bushfire burning through national parks north-west of Sydney are being told take shelter because "it is too late to leave".

The blaze at Gosper's Mountain, about 40 kilometres north of Richmond, was again upgraded to an emergency level on Tuesday evening, with firefighters struggling to hold the fire line.

The NSW Rural Fire Service (RFS) warned Colo Heights residents to take shelter in a solid structure to protect themselves from the heat of the flames.

Firefighters were bracing for weather conditions to make their job harder, with southerly winds expected to cross over the fire ground during the evening.

The blaze in the Wollemi and Yengo national parks, in the Lithgow and Hawkesbury council area, has so far consumed more than 143,000 hectares of land.

Just under 60,000 of those hectares were burnt at the weekend.

Loading

It had reached emergency level on Friday, threatening several homes and a saw mill on the south-eastern side of the fire front before being downgraded to watch and act at the weekend.

Firefighters had been carrying out backburning operations close to the Colo Heights properties as well as in the surrounding areas of Wisemans Ferry, Grey Gums and Yengo Drive.

Residents in Glen Davis, Newnes, Glen Alice, Putty Valley, and Yengo Drive were advised to monitor conditions.

"Leaving early is your safest option," the RFS said.

A property destroyed at Colo Heights in the Gospers Mountain fire. ( AAP: Dan Himbrechts )

Another blaze briefly hit emergency level in Richmond Valley, in northern NSW, after unfavourable weather conditions.

It was downgraded again just before 9:00pm.

The RFS said flames were "spreading quickly" through a pine plantation and was threatening homes in the town of Whiporie.

Aircraft, including two large air tankers, were used to hold the fire lines there.

Nearly 500 homes have been destroyed and at least 1.7 million hectares have been burnt since the start of this year's bushfire season.

The smoke from the bushfires still burning smothered Sydney on Tuesday morning, leading to 60 people being treated for health complications caused by the haze.

The haze came amid total fire bans in many parts of the state, and forecasts of a heatwave later this week, with temperatures expected to climb past 40 degrees Celsius in some areas.