Australia on alert as 'catastrophic' fire threat looms

A "catastrophic" fire risk rating has been issued for Sydney on Tuesday, Australia's largest city -- the highest possible ranking for the country's emergency services.

On the official NSW Rural Fire Services website, residents in "catastrophic" fire conditions are told to make a decision about "when you will leave."

"Homes are not designed to withstand fires in catastrophic conditions so you should leave early," the website said.

A dirt track runs through burnt forest in the Kinglake region on February 12, 2009 in Melbourne, Australia. Luis Ascui/Getty Images

Up until a decade ago, the "catastrophic" fire risk level didn't even exist because the conditions were so rare. It was only after the devastating 2009 Black Saturday bushfires in the state of Victoria, in which 173 people were killed, that the category was introduced.

Macquarie University's Mel Taylor, a researcher at the Bushfire and Natural Hazards Cooperative Research Center, said a "catastrophic" risk indicated "the worst" possible conditions.

“The catastrophic fire danger rating is based on a lot of different measures – temperature, humidity, fuel, wind – being off the scale so that if a fire starts in these conditions, it will travel very quickly and firefighters will not be able to control it,” she said.

“The only thing they will be able to do is to get people out.”