Key points:
• A state of emergency has been declared in New South Wales;
• Greater Sydney, Greater Hunter and Illawarra/Shoalhaven face "catastrophic fire danger";
• A number of Sydney suburbs are close to bushland and at risk;
• 54 fires burning in NSW, 26 of them uncontained, 13 designated "watch and act";
• Cool change expected in the early evening, which could see fires change direction;
• 54 fires burning in Queensland.

A bushfire has erupted in one of Sydney's most prestigious suburbs, confirming firefighters' worst fears as flames close in on homes.

The Canoon Road fire at South Turramurra in Ku-ring-gai was spreading quickly but firefighters have doused houses in red fire-retardant to protect them. Firefighters now have that fire under control and say the immediate threat to properties has eased.

But still the highest level of "catastrophic fire danger" is forecast for the Greater Hunter, Greater Sydney and Illawarra/Shoalhaven regions, while "extreme fire danger" is the warning for the North Coast, Southern Ranges, Central Ranges, New England, Northern Slopes and North Western areas.

The NSW Rural Fire Service has warned residents to leave homes immediately in areas facing the greatest danger, taking their pets with them.

Haze covers the Sydney Harbour Bridge.

Hundreds of schools were closed and residents were urged to evacuate woodlands for the relative safety of city centres Tuesday as hot, dry and windy weather fanned wildfires to emergency-level ferocity across Australia's most populous state.

New South Wales state is under a weeklong state of emergency, a declaration that gives the Rural Fire Service sweeping powers to control resources and direct other government agencies in its efforts to battle fires. The worst fires are expected in the state's northeast, where three people have died and more than 150 homes have been destroyed since Friday, as well as around Sydney, Australia's largest city.

Rural Fire Service Commissioner Shane Fitzsimmons said 3,000 firefighters were available to fight more than 50 fires blazing across the state. The fires are expected to worsen as winds are forecast to gust at between 70 kmh and 90 kmh later Tuesday.

"Now is the time to exercise those decisions to leave, leave early and go to safer locations, safer towns and villages or safer places in your local community, such as the shopping centres," Fitzsimmons told reporters.

"We plan for these sorts of days. But we always hope they never come," he added.

"This will only worsen throughout the afternoon as the weather conditions continue to deteriorate, and particularly as those strong winds strengthen and become more consistent across the afternoon," Fitzsimmons said.

"We are getting reports of some properties being impacted and/or destroyed in the Hillville fire, south of Taree," he said.

There has been no word of any injuries to either firefighters or members of the public at this stage.

"Please make sure you heed the messages no matter where you are, whether you are in our regions, whether you are in metropolitan areas," NSW Premier Gladys Berejiklian said earlier this morning.

"Anywhere near bushland is at threat today.

"We need to make sure every community member takes the opportunity to assess their situation and act with precaution in every possible manner."

The Greater Hunter and Illawarra/Shoalhaven areas, including Wollongong to the south of Sydney, have also been given a catastrophic fire danger rating for Tuesday — as bad as it gets.

Andrew Gissey, the former deputy chief of the Victorian SES, told Sky News that residents should be decisive and, if necessary, leave their homes.

"Having a foot in both camps is very dangerous. Thinking that you're going to stay, see what it turns out like, whether or not you can defend your home and see how it goes. That's a very dangerous mindset to take into today. If you do get into trouble, it might just be too late," he said.

None of the fires were rated as emergencies in the morning. But by early afternoon, emergency warnings applied to seven blazes that were burning out of control in the fire-ravaged northeast.

More than 600 schools and technical colleges were closed because they are close to woodlands at risk of fire.

The Australian fire season, which peaks during the Southern Hemisphere summer, has started early after an unusually warm and dry winter.

More than 1 million hectares of forest and farmland had already burned across the state this fire season, more than three times the 280,000 hectares that burned during the entire last season, Fitzsimmons said.

- text by AP, news.com.au