Authorities declared a state of emergency across a broad swath of Australia's east coast on Monday, urging residents in high risk areas to evacuate ahead of looming "catastrophic" fire conditions.

Bushfires burning across New South Wales (NSW) and Queensland states have already killed three people and destroyed more than 150 homes. Officials expect adverse heat and wind conditions to peak at unprecedented levels on Tuesday.

Bushfires are a common and deadly threat in Australia's hot, dry summers but the current severe outbreak, well before the summer peak, has caught many by surprise.

"Everybody has to be on alert no matter where you are and everybody has to be assume the worst and we cannot allow complacency to creep in," NSW Premier Gladys Berejiklian told reporters in Sydney.

The country's most populous city has been designated at "catastrophic fire danger" for Tuesday, when temperatures as high as 37 degrees Celsius (98.6 degrees Fahrenheit) are forecast to combine with powerful winds for potentially deadly conditions. It is the first time Sydney has been rated at that level since new fire danger ratings were introduced in 2009.

Home to more than 5 million people, Sydney is ringed by large areas of bushland, much of which remains tinder dry following little rain across the country's east coast in recent months.

"Tomorrow is about protecting life, protecting property and ensuring everybody is safe as possible," Berejiklian said.

Lawmakers said the statewide state of emergency — giving firefighters broad powers to control government resources, force evacuations, close roads and shut down utilities — would remain in place for seven days.

On Monday afternoon, the fire service authorised use of the Standard Emergency Warning Signal, an alarm and verbal warning that will be played on radio and television stations every hour.

NSW Rural Fire Service (RFS) Commissioner Shane Fitzsimmons urged people to evacuate before conditions worsened, warning that new fires can begin up to 20km (12 miles) ahead of established fires.

"Relocate while things are calm without the pressure or anxiety of fires bearing down the back door," he said.

Authorities stressed that even fireproofed homes will not be able to withstand catastrophic conditions, which Fitzsimmons described as "when lives are lost, it's where people die".

More than 100 schools will be closed on Tuesday.

On Monday afternoon, rescue services were moving large animals from high risk areas, while health officials warned that air quality across NSW will worsen as winds blow smoke from the current mid-north coast bushfires south.

The fires have already had a devastating impact on Australia's wildlife, with about 350 koalas feared dead in a major habitat.