A large bushfire that has threatened homes in Sydney is likely to have been deliberately lit, Australian authorities say.

The blaze, which began on Saturday, has burnt through 2,500 hectares of land in the city's southern suburbs.

Some residents had been warned of immediate danger to their lives, but firefighters say the threat has eased.

The New South Wales Rural Fire Service (RFS) described the blaze as "highly suspicious".

"How dare anyone - if they are deliberately involved in lighting fires - endanger our firefighters and also [put] all these communities in harm's way," said Commissioner Shane Fitzsimmons.

The bushfire forced some locals to evacuate their homes on the weekend, while others were told it was too dangerous to leave.

Recent weather in south-eastern Australia has been unseasonably hot, with strong winds fanning bushfires.

Firefighters use helicopters to dump water on the blaze -- Photo: EPA

Authorities said wind speeds were not as severe on Monday, making it easier to battle the blaze.

Mr Fitzsimmons said flames travelled "right up to the back fences" of houses, but no properties had been lost.

"While the threat has eased, it is still a fairly vigilant fire ground and we want people to remain vigilant," he said.

"There is a lot of work still to be done today and maybe into tonight before we can call the fire contained."

Police have declared the fire a crime scene.

Mr Fitzsimmons said it was possible that winds could strengthen again and elevate the fire danger.

On the weekend, residents hosed down their houses and lawns to prevent embers sparking new blazes ahead of the advancing fire front.

Smoke seen from a football field in the suburb of Barden Ridge -- Photo: DYLAN LONG

Major roads separating bushland and residences were cut off in some areas, and fire services used helicopters to dump water over key points.

Pictures on social media showed firefighters and residents taking care of local wildlife affected by the blaze.

Prime Minister Malcolm Turnbull has praised the emergency response.

"It is unseasonably hot and that's a matter of obviously great concern, but we have to deal with the worst that Mother Nature can throw at us and the worse Australians are presented with by nature, the better it brings out the Australian spirit," he said.

Source: BBC