Thursday marked another ferocious bushfire day across NSW and saw Premier Gladys Berejiklian declare a state of emergency for the second time this fire season.

Conditions on Saturday are "shaping up to [bring] a worse fire threat" again, the Premier said.

At the peak, four fires were burning at emergency warning level, while soaring 40-plus degree temperatures and choking smoke covered greater Sydney on what was the hottest day of the summer so far.

Read more: Rolling coverage of the Bargo emergency here

For the state's volunteer firefighters it was among the most trying days of the fire season, which has so far destroyed almost three million hectares in 105 days.

Three firefighters were seriously injured when they were overrun by fire near Bargo, while "shattered" volunteers learned many among them had lost their own homes as they fought to protect the homes of others.

"They're absolutely shattered...they are devastated by loss, no matter what, but it just goes that little further when it's your own home or the colleague you've got sitting on the fire truck next to you," said NSW Rural Fire Service Commissioner Shane Fitzsimmons.

"[It's] another very emotional, very draining day for our firefighters, with the conditions that they're facing."

In the areas of Bargo, Balmoral, Buxton at least 20 structures were lost, including at least 12 homes.

NSW will usher in Christmas day under a state of emergency next week, after Premier Gladys Berejiklian declared extraordinary powers would be in place for seven days, ahead of worsening fire conditions this weekend.

"The biggest concern over the next few days is the unpredictability, with extreme wind conditions [and] extremely hot temperatures," Ms Berejiklian said, just hours before the mercury hit 39.3 degrees in the Sydney CBD. In Penrith it reached 41.3 degrees by 12.47pm.

The four emergency warning level fires on Thursday stretched from the Hawkesbury to the Shoalhaven. Among the most volatile was the Green Wattle Creek fire, which is more than 150,000 hectares in size and is burning 15km south-west of Warragamba Dam Wall in the Blue Mountains National Park.

The Green Wattle Creek Fire, along with the Currowan fire, burning between Batemans Bay and Ulladulla, and the Gospers Mountain "mega fire," all had the potential to threaten lives and property, RFS Commissioner Shane Fitzsimmons said on Thursday.

"You've got this relentless onset of hot, dry, windy conditions and, until we see a meaningful reprieve in weather ... the reality is they're not going out," he said.

"We're up around 8500 fires so far this season. So that's the sort of challenge that firefighters and emergency services personnel are facing every day, and we've seen it day in, day out, week in, week out, month in, month out now since winter."

He said the community of Tahmoor could likely be impacted later on Thursday evening as a southerly change pushed the Green Wattle Creek fire towards the township.

It was the same fire which overcame a crew of five volunteers near Bargo. Two male firefighters, aged 36 and 56, were airlifted to hospital where they would be intubated after suffering face and airway burns, as well as burns to their arms, elbow, upper chest and leg.

A third person, a 28-year-old female from the same crew was taken by road ambulance to hospital suffering some minor burns and smoke inhalation.

"This particular crew, a crew of five, were overrun by fire. Enveloped by fire," Commissioner Fitzsimmons said.

Under the state of emergency Commissioner Fitzsimmons has extraordinary powers including the ability to evacuate properties, close roads, pull down infrastructure at risk of collapse and order the shutdown of essential utilities such as electricity, gas, oil and water.

The powers also include allowing the commissioner to direct any government agency to conduct, or refrain from carrying out its functions.

A combination of RFS-dedicated aircraft and Defence Force helicopters were deployed on Thursday, scouting ahead of fire fronts to alert people and provide rescues or extractions where needed.

At firefronts around the state crews battled fast-moving flames that topped tree canopies with the added danger of exploding gas tanks at properties in some parts.

In the Blue Mountains town of Bilpin active fire was raging on both sides by the afternoon, threatening a number of properties as it gained strength under westerly winds.

Close by Andrew Xiberras kept watch over the fire from his Kurrajong Heights property. He said he was determined to defend his family home, where all three of his children were born.

"It's a bit nerve-wracking. I'm trying to keep calm," he said, as he prepared a pump and 3000 litres of water.

By Thursday evening the Currowan, Tianjara and Kerry Ridge fires all remained at "watch and act" levels.

While fire conditions are expected to worsen by Saturday, conditions are expected to ease on Friday.

The Bureau of Meteorology has issued a severe fire danger warning for the Central Ranges for Friday and a very high fire danger warning for the Southern Ranges, Northern Slopes, North Western and Southern Slopes.

National heat records could be broken again once Thursday's temperatures were analysed by the bureau, after the national average temperature reached a new high of 40.9 degrees on Tuesday, before immediately being broken on Wednesday.

In Sydney, It is now expected to reach 42 degrees in Parramatta on Saturday, and a staggering 47 degrees in Penrith, while sea breezes are expected to push bushfire smoke from coastal areas near the CBD into the west of greater Sydney.

As the weekend and Christmas break drew closer, Ms Berejiklian urged motorists and holidaymakers to avoid travelling in the vicinity of fires unless it was absolutely necessary.

"If you are told a road is closed, if you are told not to travel please take the advice, this is a state of emergency, we don't take it lightly," she said.

"We are concerned about weather conditions for a couple of additional hours today, we will take advantage of calmer conditions [on Friday] in preparation for Saturday, which is unfortunately shaping up to be a worse fire threat than today."

with Laura Chung, Matt Bungard