Smoke haze from bushfires raging in Australia spread to the capital on Sunday, as firefighters raced to contain more than 140 blazes ahead of a heatwave forecast early this week.

A firefighter is conducting back burning measures on Saturday to secure residential areas from encroaching bushfires at the Mangrove area, Australia. Photo: AFP

Australia is experiencing a horrific start to its fire season, which scientists say began earlier and is more extreme this year due to a prolonged drought and the effects of climate change.

Residents of Canberra in the country's southeast woke up to see the capital shrouded in haze Sunday, joining those in Sydney who have endured weeks of toxic air pollution caused by bushfire smoke.

Officials said favorable weather conditions had given them a chance to bring several blazes under control before the forecast return of strong winds and high temperatures Tuesday.

Among those is a "mega fire" burning across 250,000 hectares within an hour's drive of Sydney, Australia's largest city, where ash from the fires has occasionally fallen.

Firefighters are now bracing for Tuesday, when temperatures are expected to reach above 40 Celsius in parts of New South Wales (NSW) state - worst-hit by the bushfires - and gusting westerly winds are likely to fan the flames.

"Today [fire] crews will be doing what they can to consolidate and strengthen containment lines, which in some areas will include backburning," NSW Rural Fire Service spokesman Greg Allan told AFP.

But the state's Bureau of Meteorology warned that the massive fires are "in some cases just too big to put out at the moment."