The NSW Rural Fire Service (RFS) has warned it would be a "miracle" if more homes were not lost tomorrow amid a catastrophic fire danger warning across the south-eastern coast.

Key points: It is only the second time the catastrophic rating level has been issued for Sydney since the new system was introduced in 2009

It is only the second time the catastrophic rating level has been issued for Sydney since the new system was introduced in 2009 A total fire ban will be in place across the state

A total fire ban will be in place across the state It comes after NSW Premier Gladys Berejiklian declared a week-long state of emergency

It is only the second time such a warning, the highest level of bushfire danger, has been issued for the Greater Sydney area since the rating system was introduced in 2009.

Both instances have been during this year's unprecedented bushfire season in NSW, which has already seen blazes tear through 2.9 million hectares.

Tomorrow, the warning will be in place across Greater Sydney, the Illawarra/Shoalhaven and the Southern Ranges.

Temperatures are expected to peak at the mid-40-degree mark during Saturday's heatwave, with gusty winds and dry conditions exacerbating the 100 blazes burning in the state.

The Transport Management Centre said several roads across NSW would be closed tonight due to fires and backburning.

The Great Western Highway between Little Hartley and Blackheath was closed at 6:00pm but was later reopened due to changing wind conditions.

Roads around Blackheath were closed to an emergency level fire and backburning. ( ABC News: James Carmody )

NSW RFS Deputy Commissioner Rob Rogers said the forecast of extreme weather on Saturday throughout the state would bring "major problems".

"Tomorrow is possibly going to be one of the toughest days we've had," he said.

"I know we've had a lot of difficult days over this fire season and we had a really tough day yesterday. But tomorrow, from a fire point of view, could be even tougher."

At least 800 buildings have so far been destroyed during this year's bushfire season and Mr Rogers said the number was likely to increase over the weekend.

"I think it is a fairly sure thing we will lose homes somewhere tomorrow. It will be a miracle if we didn't," he said.

Some 1,800 firefighters are working today and that number is expected to double tomorrow.

It has already been an exhausting bushfire season for NSW firefighters. ( ABC News: Brendan Esposito )

Mr Rogers said the Gospers Mountain fire was causing problems at the Lithgow end and was expected to move towards Kurrajong Heights tomorrow.

The RFS will be undertaking backburning tonight to try and protect communities around Blackheath.

Efforts will also be focused on stopping the Green Wattle blaze from crossing the Hume Highway.

The Princes Highway down towards Shoalhaven is also expected to be closed tomorrow.

Smoke plumes pouring out near Bilpin on Thursday. ( ABC News: Brendan Esposito )

Mr Rogers warned the public to make a plan and leave early to ensure their lives and the lives of others are not put at risk.

"Yesterday, we saw trained firefighters who had the right equipment and training, they got burnt and overrun in these fires," he said.

"We are very lucky we did not see serious casualties [among] the public yesterday and we don't want to see that tomorrow.

"Go somewhere else. Go to the beach, shopping centre, go somewhere else for the heat of the day and make sure you are not there."



The blaze at Green Wattle Creek is also burning at emergency level. It has already razed through 176,000 hectares of land.

Two firefighters Geoffrey Keaton, 32, and Andrew O'Dwyer, 36, died while helping fight the blaze yesterday.

On Thursday NSW Premier Gladys Berejiklian declared a state of emergency for seven days.