Factbox: Key facts about Australia's bushfires

A ferry passes in front of the Central Business District as smoke from bushfires shroud Sydney

(Reuters) - Australia's east coast faces a "catastrophic" fire threat on Tuesday, authorities have warned.


THREAT LEVEL

The catastrophic rating was introduced in 2009 following the so-called Black Saturday bushfires that killed 173 people. This is the first time it has been declared for Sydney.


Warning comes as temperatures in Sydney are expected to hit 37 degrees Celsius (98.6 degrees Fahrenheit), with strong winds elevating the risk.


NSW Rural Fire Service Commissioner Shane Fitzsimmons says catastrophic conditions are "when people die". No home, even a fireproofed one, is able to withstand the effects, Fitzsimmons warns.


Sydney, home to more than 5 million people, is ringed by large areas of bushland, which back onto residential areas.


AUSTRALIA'S HISTORY OF BUSHFIRES

High temperatures and low rainfall make bushfires a natural hazard. Natural tree oils in native eucalypt forests can fuel fires.


Common causes are lightning strikes or humans, who drop cigarettes or light blazes deliberately. Australia’s science agency says controlled burns, where fires are deliberately lit to clear out excess dry vegetation that can fuel wildfires in high-risk areas, are key to management. But backburns can spiral out of control.


Bushfires have killed hundreds of people in recent decades.


The deadliest bushfires in Australian history, the Black Saturday fires of February 2009 in the state of Victoria, would have had a catastrophic rating under the system that was imposed after they killed 173 people.


As well as human impacts, long-term bushfire effects include the loss of livestock and animal habitats, reduced soil fertility lasting decades and contamination of water catchments with ash and debris.


(Reporting by Colin Packham; editing by Jane Wardell)