The NSW Rural Fire Service (RFS) said firefighters and waterbombing aircraft had succeeded in slowing the spread of a blaze burning near the "mega fire" north of Sydney.

Key points: The RFS said dry thunderstorms were expected in the afternoon, prompting fears lightning will ignite more blazes

The RFS said dry thunderstorms were expected in the afternoon, prompting fears lightning will ignite more blazes Total fire bans have been declared in nine NSW regions today

Total fire bans have been declared in nine NSW regions today At least 2.7 million hectares of NSW has been burnt since the beginning of this year's bushfire season

An emergency warning was issued for the Three Mile fire burning to the west of Gosford on the Central Coast, with people in the areas of Kulnura and Central Mangrove warned it was "too late to leave".

It is understood that about 100 rural properties are in the affected area.

It has since been downgraded to watch and act and residents have been told to monitor conditions.

However, there are fears it could join up with the Gospers Mountain "mega fire" to the west in the coming days.

A southerly influence had brought cooler conditions to firegrounds in the afternoon, with temperatures dropping as much as 10 degrees in a matter of minutes, but it has also brought increased winds.

Earlier, temperatures in Penrith in Sydney's west had topped 40 degrees but the RFS said it would be some hours before challenges lessened for firefighters.

RFS Commissioner Shane Fitzsimmons said it would be a "very complex, very difficult day" for firefighters who had taken advantage of cooler weekend conditions to conduct "critical backburning".

The RFS said at 5pm there were 83 fires burning with 40 of them uncontained.

The fire reach across the state is stretching some 11,000 kilometres, the RFS said.

A man hoses down his gutters in Oakdale, near Sydney, today. ( ABC News: Harriet Tatham )

Mr Fitzsimmons said thunderstorms were expected, but there would be little rain.

"That means a potential for lots of lightning," he said.

Nine areas across NSW have total fire bans in place today, with more than 2,000 personnel and 100 aircraft fighting fires.

"All fires ... are going to be of concern to us, but there are some that are much closer and with greater potential to impact on more densely populated or highly populated areas," Mr Fitzsimmons said.

The RFS said at least 2.7 million hectares, with a perimeter of 19,235 kilometres, had been burnt so far this bushfire season.

One of the blazes, dubbed the "mega fire", had a front of more than 60 kilometres and was a result of five separate fires which merged north of Sydney on Friday.

Firefighters on the Central Coast are battling smoky conditions. ( Facebook: NSW RFS )

The NSW Premier Gladys Berejiklian said today had given NSW a "taste" of what NSW will face this summer.

"We can't underestimate what it can be like over summer," she said.

"As the conditions get hotter and drier, and also the wind picks up, this again is giving us a taste of what we're likely to experience in the next few months."