"It's only a matter of hours before we see a flare-up in fire activity and a breach of these tenuous containment strategies," said Shane Fitzsimmons, Rural Fire Service (RFS) commissioner, in Sydney earlier in the day.
The wildfires stretch along a nearly 1,000-mile line in New South Wales, from the far north of the state south of Brisbane -- which lies just across the Queensland border -- to east of Canberra, the country's federal capital. Fires in the Blue Mountain range west of Sydney are a particular worry as rough terrain has impeded firefighting efforts.
"If our strategies don't work and weather materializes tomorrow that is forecast ... it could be absolutely devastating," said Rural Fire Service (RFS) Deputy Commissioner Rob Rogers in New South Wales on Tuesday. "We lost a couple hundred homes the other day -- we could get worse losses than that." Wednesday is expected to be "about as bad as it gets," added Fitzsimmons.
With high temperatures, low humidity and strong winds forecast, fire officials have urged residents to evacuate from the path of the fires. "On days like today, minutes really matter," Fitzsimmons said.
At least one death has been reported. A 63-year-old man died of a suspected heart attack Friday while defending his home against a blaze on the New South Wales Central Coast, the Australian Broadcasting Corporation, or ABC, reported.
'Eerie' skies over city of Sydney
Some urban areas that border the Australian bush have been devastated by the fires, which have destroyed more than 200 homes in the Blue Mountains region since Thursday.
Scores of fires now have burned 25,800 hectares (310,859 acres) -- an area greater than the size of Los Angeles. On Tuesday, officials said they were bringing in 1,500 firefighters to join the more than 1,000 emergency crews already on the ground.
At Blue Gum Forest inside the Blue Mountains National Park, firefighters were doing controlled burns -- setting fires in the bush ahead of the frontline of the blazes to eliminate tinder and slow the progress of the fires. Instead of cloud-shrouded mountaintops, the valleys of the region were covered with a dense layer of smoke.
Fears are still high that fires could join to form a "mega-fire" and endanger heavily populated areas of this popular natural retreat west of Sydney.
Most of the firefighters in the area were volunteers who've left their ordinary jobs to put out the fire, pouring in from different parts of Australia.
"There's a sense of community, doing what they can -- you can get a sense of resilience, strength, and a deep sense of pride of each brigade pulling their weight," said CNN's Robyn Curnow, reporting from the Blue Mountains.
The state of emergency issued by authorities gives firefighters and police the authority to carry out measures such as cutting off water, power and gas and ordering mandatory evacuations of areas at risk.
A total fire ban is in place for the Greater Sydney region until further notice, officials have said, meaning no fire may be lit in the open, and all fire permits are suspended. The fires have spread a cloak of smog over Sydney in recent days.
The bush fires in the area spread out of control Thursday amid high temperatures and powerful winds. Emergency officials said the region is emerging from a very dry winter and has had little rain in recent months.
The causes of the Blue Mountains fires are still under investigation -- officials are looking into whether one major blaze was caused by a military training exercise.
Police said Monday they had arrested two boys, age 11 and 15, over two earlier bush fires in the Port Stephens area, more than 100 kilometers (60 miles) north of Sydney, that began October 13. One of the fires they are accused of starting burned more than 5,000 hectares (12,000 acres) in the surrounding area.
The 11-year-old has been placed under house arrest, charged with two counts of intentionally causing fire and being reckless as to its spread, NSW Police Force said. The child is scheduled to appear in court next month where he is expected to plead not guilty to the charges, CNN affiliate Sky News reports.