Scorching temperatures, bone-dry bushland, high winds and severe lightning storms have combined to create a worst bushfire threat in 100 years.

The volatile cocktail of conditions forced authorities in New South Wales (NSW) to issue "catastrophic" fire warnings and impose a total fire ban across most of the State.

As Sydney awaits a high of 40C on Sunday, the message from authorities is clear: this is only the beginning, The Sunday Telegraph reports.

"These are conditions that haven't been seen for 100 years," Premier Nathan Rees said on Saturday.

Residents have been warned that worse is to come, with hot, dry weather expected to continue over summer.

At least 66 fires are burning throughout NSW, including the Blue Mountains, the Hawkes bury region and the north. As of late Saturday, no private property or lives had been lost.

Dry lightning storms on Friday night and 45km/h winds have put virtually all of NSW under threat and the Rural Fire Service on high alert.

Fire officials fear more fires may break out on Sunday as conditions worsen. Rural Fire Service Commissioner Shane Fitzsimmons said anything was possible.

As many as 1,100 firefighters and 380 tankers worked around the clock to contain blazes, with thousands more on standby Sunday in case conditions worsen, local media reports.

"We don't know where the next fire is going to start and, clearly, we will be monitoring the existing fires, in light of these elevating weather conditions," Fitzsimmons said.

Bureau of Meteorology forecasting chief Rob Webb said the late- spring heatwave was just the beginning, with unusually high summer heat possible for NSW in January and February.

"Tomorrow, we expect temperatures to rise back to 40C in Sydney, and much of north-eastern NSW will see temperatures towards the mid-40s."

"Ahead of a strong front approaching, we will see wind speeds get up to 40km/h. This will put pressure on fires already started by lightning strikes." he said.

The premier said most of the fires were the result of dry lightning strikes. People in fire-prone areas are urged to check radio updates and RFS website.