New South Wales residents are being warned to prepare for one of the most serious fire threats in the state's history today, with conditions in some areas rated as "catastrophic".

A total state-wide fire ban has been issued, with a catastrophic rating in place for the Illawarra, Shoalhaven and Southern Ranges.

Many other areas are expecting extreme danger levels.

Temperatures are expected to soar above 40 degrees in most parts of the state today, including in Sydney.

More than 100 fires are burning across the state; around 21 of those are not under control, including a large grassfire at Oura near Wagga Wagga.

Overnight 1500 firefighters worked to bring those blazes under control ahead of today's expected high temperatures and strong winds.

A firefighter, meanwhile, has been flown to Sydney Hospital in a stable condition after he was burned while battling a blaze near Gunning, north of Canberra.

National parks, reserves and state forests across the state will be closed and anyone in parks should immediate prepare to vacate camping areas.

Premier Barry O'Farrell says record rains have produced large fuel loads which increase the risk of fire, combined with record temperatures and high winds.

"[It] is not going to be just another ordinary day," he said.

"[It could] be perhaps the worst fire danger day this state has ever faced."

Mr O'Farrell has warned Sydneysiders they will not be spared from the heat.

"If Sydney reaches 43 [today], it will only be the third time in the history of record keeping that the temperature in Sydney has been that high," he said.

The Premier says people must have their bushfire survival plans in place, as complacency could cost lives.

Today's conditions will be the worst fire fighters have ever faced, but Mr O'Farrell says the state is up to the challenge.

"I'm confident that the state's emergency services from the Rural Fire Service to the police and others are some of the best trained in the country," he said.

"To the extent that you can predict these things.. we're as well prepared as possible."

Victorian Country Fire Association Chief Officer Euan Ferguson says help is at hand if it's needed.

He says the CFA is moving resources north to high-risk areas to help New South Wales if required.

"We'll be moving some of our heavy aircraft resources more northward," he said.

"In case we need to support new fires in Victoria or even support our counterparts up in NSW."

Meanwhile, the Premier's urging residents across the state to be on the lookout for firebugs.

"If they see people lighting fires they should call 000 or Crime Stoppers on 1800 333 000," he said.

Mr O'Farrell says people who are caught deliberately lighting fires face will stiff fines and harsh jail sentences.

"Just lighting a fire during a total fire ban can attract fines of $2200, increasing to $110,000 if property is damaged."

'Destruction is likely'

The RFS map showing fire danger areas for Tuesday. ( NSW Rural Fire Service )

Rural Fire Service (RFS) Commissioner Shane Fitzsimmons says people should be alert to any danger.

"Catastrophic fire danger ratings are the worst you can get. We are talking about the most extreme fire behaviour - destruction is likely," he said.

"We will see ember showers likely to be thrown 10 kilometres, if not 20 kilometres ahead, under the extraordinary conditions being forecast."

Commissioner Fitzsimmons says communities should prepare for the worst.

"We are looking at widespread areas of New South Wales likely to experience very high, severe, and even catastrophic conditions," he said.

Commissioner Fitzsimmons says people need to monitor their local conditions.

"They need to listen to radio, they need to watch TV, they need to refer to the website, they need to access social media tools such as Facebook and Twitter, do not rely on one source for all your information."

Thousands of firefighters and about 70 aircraft have been placed on standby across the state.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 3 minutes 46 seconds 3 m 46 s NSW braces for worst ever day of bushfire danger

The Commissioner says people camping and holidaying on the state's south coast, especially around Nowra, should re-think their plans before the catastrophic conditions take hold.

The NSW Ambulance Service is warning people to take care during the predicted heatwave.

Acting Chief Superintendent Ian Johns says the heat will be particularly bad for the elderly and those with underlying medical conditions.

He says everyone should plan to drink plenty of water.

"The paramedics will be ensuring that their vehicles have plenty of intravenous fluid, also plenty of bottled water too," he said.

"But whilst we can come to you once you're ill, the plan and the message is prepare yourself so you don't require our services.

"Drink water and plan your day's activities."

Health Department spokesman Dr Richard Broome says it is important to postpone any unnecessary activities.

"Hot weather and heat puts a great deal of strain on the body," he said.

"In order to keep cool we have to pump blood to the skin. People who are old or have chronic diseases, their heart might not work quite so well and the strain can precipitate illness like heart attacks."