"Catastrophic" and "extreme" fire danger is forecast for parts of NSW on Tuesday, November 12.
There is a total fire ban in place for all of NSW on Monday, November 11 and Tuesday, November 12.
NSW RFS has said they will not be able to contain fires on the north coast and in northern NSW ahead of the high-risk conditions.
"Under these conditions, these fires will spread quickly and will threaten homes and lives," and RFS spokesperson said.
"These conditions will be as bad, if not worse, than those experienced on Friday as they will be across a much broader area including large population centres like Sydney."
A combination of high temperatures, strong winds and low humidity make the conditions so dangerous.
The Illawarra and Shoalhaven are predicted to experience extreme fire danger - one level below catastrophic. This is subject to change as forecasts are updated.
Catastrophic is the highest level of bush fire danger. Homes are not designed to withstand a fire under these conditions.
The RFS recommends those in areas experience catastrophic conditions: "Avoid bush fire prone areas. A safer area may be a large town or city, shopping centres or facilities well away from bushland areas. If you are unable to leave, identify a safe location which may be nearby. This may include a Neighbourhood Safer Place."
This is the first time since new Fire Danger Ratings were introduced in 2009 that catastrophic fire danger has been forecast for Sydney.