The NSW Royal Fire Service (RFS) is urging residents to be extremely vigilant as the state's heatwave creates "the worst possible" fire conditions.
Addressing the media on Saturday, the NSW RFS Commissioner Shane Fitzsimmons said the state has never seen conditions so severe.
"It is not another summer day, it is as bad as it gets ... They are off the old conventional scale," he said.
Catastrophic conditions have been forecast for the Greater Hunter, Central Ranges and North Western areas.
The NSW RRS is pleading with people in areas classified as "catastrophic" to leave this evening or very early tomorrow morning, otherwise their safety "cannot be guaranteed".
When a fire start in areas classified as "catastrophic" -- unless emergency services are in the area early -- the blaze is unlikely to be contained. The commissioner warned that though emergency messages will be sent, it is likely be too late to evacuate.
"History will show too many lives are lost when people are leaving at the last minute," Commissioner Fitzsimmons said.
"The only safe place to be is not in these areas ... fires will start easily, and spread quickly, this might happen before anyone is aware.
"The risk is real, complacency and lethargy kills."
Dozens of national parks have closed across the state, as emergencies services prevent people from camping and bushwalking in fire prone areas.
Acting NSW Regional Director Stephen Lellyett said the conditions have been caused by a stationary mid-level ridge over central Australia had caused a build-up of heat over the interior of the continent over the last few weeks.
"The system will lead to increasing wind speeds in conjunction with the extremely hot temperatures," he said.
"The forecast Catastrophic fire weather conditions on Sunday in the upper Hunter and fringes of adjoining districts are rare in NSW. Sunday is shaping up to be the worst fire weather day so far of the season."