Australia's largest city, Sydney, is blanketed in smog as more than 90 bushfires blaze across the state of New South Wales, intensified by the country's worst drought in decades.

Phil Koperberg, head of the state's rural fire service, said 32 of the fires were out of control and firefighters were exhausted.

The slightest breeze is now causing fires to breach containment lines

Phil Koperberg

"We've been fighting the same fires in many parts of the state for many weeks with conventional containment strategies failing time and time again because the bush and the forest is so dry," he told Australian television's Channel Seven.

Conditions are not expected to improve. Weather forecasters have predicted that this weekend will be hot, dry and windy.

"Even the slightest breeze, which under normal conditions wouldn't concern us, is now causing fires to breach those containment lines and once having done so they are burning with great velocity and are difficult to suppress," Mr Koperberg said.

Drought blamed

The bushfire season usually runs from December to February, when summer's heat sends temperatures above 40 degrees Celsius (104 degrees Fahrenheit).

But this year the fires began months early, thanks to a drought affecting 70% of Australia.

Nobody has been injured by the flames and on Friday there were no fires immediately threatening Sydney.

But Mr Koperberg told residents across the state to remain vigilant.

Firefighters have been battling blazes since June

Just hours after his warning, a new fire broke out at a nature reserve behind homes in the Blue Mountains about 90 kilometres (55 miles) west of Sydney.

"Firefighters are doing property protection, standing around houses and making sure fire doesn't get them, hosing houses down," local fire service spokeswoman Meeka Bailey said, adding that none of the homes were immediately threatened.

Mr Koperberg said some of the fires were the work of arsonists.

About 2,000 firefighters were battling blazes on Friday.

Last year, dozens of homes were destroyed by hundreds of fires around Sydney over Christmas and New Year.