WHAT DOES THE RATING MEAN?

* The fire danger for Tuesday has been rated "catastrophic".

* "It's where people die", says NSW RFS Commissioner Shane Fitzsimmons.

* Residents are being warned fires under these conditions are hard to suppress and homes will burn.

* People are being told to leave for their survival now.

* This is the first catastrophic declaration for Sydney since a new rating system came into effect 10 years ago.

WHICH AREAS ARE AFFECTED?

* Greater Sydney: 35 local government areas running from beachside suburbs like Manly to the Blue Mountains in the west. The danger areas are listed below.

* Sydney West and Inner West: Burwood, Canada Bay, Strathfield, Blacktown, Campbelltown, Canterbury-Bankstown, Cumberland, Fairfield, Liverpool, Parramatta Penrith, Sydney (includes some inner west suburbs).

* Sydney East: Randwick, Sydney, Waverley, Woollahra.

* North and North West: Camden, Central Coast, The Hills, Hawkesbury, Hunters Hill, Ku-ring-gai, Lane Cove, Mosman, North Sydney, Northern Beaches, Ryde, Willoughby.

* South and South West: Georges River, Kogarah, Sutherland.

* The Hunter: Cessnock, Dungog, Lake Macquarie, Singleton and the Upper Hunter.

* The Illawarra and Shoalhaven areas south of Sydney.

HOW DANGEROUS WILL IT BE?

* Australia's worst bushfire tragedy was the Black Saturday fires of February 2009, which killed 173 people and destroyed thousands of homes. It would have carried a catastrophic rating under the present system.

* The current fire danger flags a similar level of destructive force.

* The catastrophic rating for Greater Sydney is "unprecedented" but there have been one or two warnings for other parts of the state in the past decade.

* "We got to keep reminding ourselves that catastrophic is off the conventional scale," Mr Fitzsimmons says.

Australian Associated Press