Firefighters are preparing to defend the tiny town of Cassilis in the central west from a fire front driven by increasing winds, ahead of a cool change expected to break the state's historic heatwave later this evening.

The Rural Fire Service had earlier directed people from outlying areas to head for shelter in Cassilis, a town of about 300 people, but with the fire approaching, the RFS now says it has crews in place to defend the town should the fire hit.

Bushfires burning in four parts of the state have triggered five emergency warnings, with residents told to shelter as the fires approach.

The Rural Fire Service has issued warnings for fires burning:

• at Pappinbarra, west of Port Macquarie; 
• near the towns of Uarby and Turill, north of Mudgee;
• at Kains Flat, north-east of Mudgee;
• near Boggabri, in north-western NSW;
• at Dondingalong, near Kempsey.

Shortly after 3pm, the RFS reported that the fire at Kains Flat, north-east of Mudgee, was burning towards homes. At 3.20pm, it said the fire north of Mudgee was burning east towards Cassilis and had breached containment lines.

"People in the area of Uarby, Turill and Cassillis should seek shelter as the fire front approaches. It is too late to leave."

Anyone east of Cassilis has been advised to leave now towards Merriwa, and anyone north of Ulan should move south towards Mudgee.

Near Port Macquarie, two fires were reportedly spreading quickly towards the village of Beechwood at 2.45pm, with people living near Pappinbarra Road in Hollisdale, Lower Pappinbarra and Beechwood told to prepare for the fire front.

Two further emergency warnings were issued at 4.20pm for a grass fire near Binalong Road, south of Boggabri, and for an out-of-control bushfire near Spring Hill Road in Dondingalong.

Earlier in the day, the RFS said there were unconfirmed reports of a house lost in the fire north of Mudgee, known as the Sir Ivan fire.

Emergency alert telephone messages were being sent to people in the area.

"The fire is burning in catastrophic fire conditions," the RFS said. "In these conditions, the fire will spread quickly. It will be difficult for firefighters to contain the fire."

The Golden Highway has been closed due to smoke and is expected to remain closed for the remainder of today.

Soaring temperatures across much of the state have led to warnings of catastrophic fire conditions. In Walgett, the temperature has hit 46 degrees.

As NSW faces the "worst possible fire conditions" in its history with 'extreme' and catastrophic' warnings in place across large slabs of the state, RFS Commissioner Shane Fitzsimmons said the situation was as "bad as it gets" and warned it was set to get worse on Sunday when winds are expected to sweep through scorched parts of mid to northern NSW. 

"To put it simply [the conditions] are off the old scale," he said. "It is without precedent in NSW". 

As of 11am, the RFS reported 76 bush and grass fires across NSW with 26 not yet contained. Deputy Commission Rob Rogers told ABC news:  "It's going to be a really tough day." 

Concerns are also held around the mid-north coast around Taree and Wauchope. A number of fires are burning to the east of Buladelah.

Deputy Commissioner Rogers warned residents in areas listed as 'catastrophic' to leave their homes.

"We've got everything we can possibly need to be ready for this but obviously in conditions forecast like this, we can't guarantee to save everybody, we can't guarantee to save every house, we can't even guarantee to have a fire truck at every fire.

"People just need to focus on their own safety today."

A dramatic graphic image of the threat released by the RFS on Sunday shows a great swathe of the state covered in red, representing a 'catastrophic' or 'extreme' threat.

The area stretches from the lower central west plains taking in Dubbo, Parkes and Wellington, right up to the Queensland border in the north western region taking in Moree and Walgett.

The extreme threat stretches to the east taking in Gunnedah, Tamworth and the coastal Kempsey, Coffs Harbour region.

On Sunday morning, the Southern Ranges and Easter RIverina were upgraded to very high fire danger.

Commissioner Fitzsimmons said "catastrophic" fire ratings had been issued only once before in NSW - in 2013 - since national standardised ratings were introduced in 2009. Sunday's catastrophic fire rating will stretch from Dubbo to Coonabarabran to Port Stephens, affecting the Central Ranges, North Western NSW and the Greater Hunter.  

He said strong winds were affecting fire fighters' ability to tackle the fires from the air with much of the fleet grounded.

Commissioner Fitzsimmons said conditions in some parts of NSW could be worse than Victoria's Black Saturday fires, Australia's worst ever fire disaster which claimed 173 lives in 2009.

He warned residents to prepare themselves. "We can't guarantee that a warning or telephone message will occur for everyone that comes under threat," he said. Text message warnings were being sent to residents in the Hunter Valley region on Saturday night.

Sydney's west recorded its hottest ever day, hitting 46.9 degrees at Penrith, according to the Bureau of Meteorology, eclipsing its previous hottest of 46.5 degrees in January 2013, while Richmond touched the 47 degree mark. 

On the coast, Saturday's top of 36.1 degrees made it a record 11th day above 35 in Sydney this summer, while in the state's west, the small township of Ivanhoe, population 200, reached a stifling maximum of 47.6 degrees; the second hottest February temperature ever recorded in NSW.