NSW is facing the "worst possible fire conditions" in its history with a "catastrophic" warning in place across large slabs of the state after a heatwave smashed temperature records on Saturday.

Rural Fire Service (RFS) Commissioner Shane Fitzsimmons said the situation was as "bad as it gets" and warned it was set to get worse on Sunday when winds are expected to sweep through scorched parts of mid to northern NSW.

"To put it simply [the conditions] are off the old scale," he said. "It is without precedent in NSW".

Commissioner Fitzsimmons said "catastrophic" fire ratings had been issued only once before in NSW - in 2013 - since national standardised ratings were introduced in 2009. Sunday's catastrophic fire rating will stretch from Dubbo to Coonabarabran to Port Stephens, affecting the Central Ranges, North Western NSW and the Greater Hunter.