Video report by ITV News Correspondent Liz Summers
Authorities are warning high winds that have been forecast will impact the country's largest city and could also spread the flames even faster.
New South Wales Rural Fire Service (NSW) said high temperatures and strong winds are making conditions even more dangerous, and are expected to peak on Tuesday.
"Catastrophic fire danger has been declared for Tuesday in Sydney and Hunter areas," they said.
The NSW Rural Fire Service added: "We are entering an extremely dangerous period of fire danger across NSW - because of the large amount of fires already burning, and increased fire danger ratings.
"Know the fire danger and what you will do, especially if you're in an area of Catastrophic fire danger."
It is the first time Greater Sydney has received this level of warning since the ratings system was introduced a decade ago.
Australian Prime Minister Scot Morrison said: "We're not out of this yet. There is a long way to go and Tuesday is looking a lot more difficult.
"And that is not only true here in New South Wales."
He added: "We know of the similar types of conditions we are seeing in Western Australia on Tuesday as well."
"So while we're seeing a very outstanding state effort here in New South Wales, there is a coordinated effort going across the country in the other places where there is danger"