Every national park in NSW is to be closed as the state faces its worst fire danger day in history with severe, extreme and catastrophic conditions.

A total fire ban will be in place across NSW on Tuesday with temperatures in the state's far west predicted to hit 45C, while the mercury is forecast to climb to 43C in Sydney - the third highest on record.

"Having had record rain ... we are about to have record dangers," Premier Barry O'Farrell told reporters in Sydney on Monday.

"Tomorrow will be perhaps the worst fire danger day the state has ever faced."

He urged those living in Shoalhaven, the Illawarra and the southern ranges to be prepared, with a catastrophic fire danger warning in place.

All state forests and national parks in NSW will be closed on Tuesday due to fire risk and everyone in the parks has been told to leave as soon as possible, while residents of high risk areas have been urged to evacuate.

"We are expecting horrendous conditions tomorrow and in the interests of public safety as well as the safety of fire fighting personnel we want the parks empty of people until the danger of bush fires has passed," National Parks and Wildlife Service Director of Conservation Operations Bob Conroy said.

Where appropriate, nursing homes and child care centres will be evacuated in fire risk zones.

NSW Fire Service Commissioner Shane Fitzsimmons also said it was "not recommended" that people living in catastrophic fire danger zones remain behind to protect their properties.

"We are talking about fire danger right off the conventional chart," he said.

"Some fires will start and spread so quickly that there will be little or no time to issue a warning."

An abundance of fuel and strong north-westerly winds of between 60km/h to 80km/h have heightened fire dangers in NSW, Mr Fitzsimmons said, with about 90 fires burning across the state on Monday.

Thousands of firefighters and up to 70 aircraft are on standby around the state.

Acting Deputy Police Commissioner and State Emergency Operation Controller Mick Fuller said police would be targeting suspected or known arsonists on Tuesday and he asked people to be on the lookout for suspicious behaviour.

A firefighter injured while battling a blaze near Gunning north of Canberra on Monday was taken to hospital, the RFS said.

Meanwhile, an emergency warning was issued to residents of Oura near Wagga Wagga in southern NSW as firefighters battled a nearby fast-moving grassfire but that was downgraded to a watch and act about 4pm (AEDT).

The fire danger accompanies heatwave conditions in many parts of NSW, with authorities urging people to take care to avoid heat-induced illnesses, to be careful in the water and to watch out for the elderly.