People in a large band of NSW towns have been urged to leave their homes following days of extreme heat that's led to unprecedented and catastrophic fire conditions.

Sunday's tinder dry conditions are being likened to those in the lead-up to Victoria's Black Saturday blazes, which killed 173 people in 2009.

"It's not another summer day, it's not another bad fire weather day. This is as bad as it gets," RFS Commissioner Shane Fitzsimmons said of the conditions.

"To put it simply, they're simply off the old conventional scale. We used to stop our fire danger ratings at 100. We're talking indices tomorrow up over 150, 160, which is quite extraordinary," he said.

Extreme to catastrophic fire danger ratings are forecast for centres including Dubbo, Coonabarabran and Narrabri in the north through to the Hunter Valley and the coast at Port Stephens on Sunday, Mr Fitzsimmons said.

"Everyone in those areas should be considering moving to a safer locations - either contemplating that tonight ... or indeed early tomorrow morning before the risk materialises," he said today.

It comes after days of sweltering conditions, with more than 50 towns and centres in NSW surpassing 40 degrees on Saturday.

While the RFS would do everything that it could to prepare for, respond to and prevent destruction, Mr Fitzsimmons warned that there were no guarantees.

"We simply cannot guarantee that a fire truck will be at every home or at every property under these conditions," he said.

"We will not guarantee that a warning, a telephone message or a knock on the door will occur for everyone that comes under threat tomorrow."

As the RFS chief spoke, 49 fires were burning across NSW, 17 of them uncontained.

About 300 firefighters were on the ground and thousands more were on stand-by.