More than 200 homes have already been destroyed and another 120 damaged by the wildfires, which broke out across New South Wales state in unseasonably warm weather last week, fanned by extremely high winds.
The worst of the blazes, in the Blue Mountains west of Sydney, plunged the city last week into an eerie midday darkness as plumes of smoke and ash filled the sky.
One man has died so far trying to protect his property.
"This is not an action taken lightly... But it's important the Rural Fire Service (RFS) and other emergency services have the powers and the resources they need to combat this threat," said O'Farrell.
"We are planning for the worst but hoping for the best."
Officials issued dire predictions about the worsening weather forecasts through to Wednesday, with a RFS spokesman saying there was the "very real potential that these three fires -- (one) in Lithgow and the two in the (Blue) Mountains -- could form as one fire over the next couple of days".
Sydney's suburban outskirts are just across the Nepean River from the foot of the mountains. Embers jumped its banks on Thursday, starting a fire at Castlereagh near Penrith.
Some 76,000 people live in the Blue Mountains while the population of Sydney is 4.4 million.
NSW RFS Commissioner Shane Fitzsimmons said there would be several extremely difficult days ahead for fire crews, with temperatures set to soar and a return to dangerously high winds.
"If you are to draw a parallel, and it's always dangerous to draw a parallel, at best you'd be going back to time periods in the late 60s."
"The reality is, however, these conditions that we're looking at are a whole new ball-game and in a league of their own.
