SYDNEY - More than 500 firefighters, dozens of fire trucks and 15 aircraft battled a major bushfire south-west of Sydney on Sunday (April 15) to try to protect scores of homes threatened by the blaze.

Australian Broadcasting Corp online reported that some homes had been affected. It said the New South Wales Rural Fire Service (RFS) couldn't immediately confirm the level of damage.

But the fire service said the blaze had burned about 2,500 ha around Holsworthy and Menai, with other nearby areas also under threat.

Fire conditions eased later on Sunday, allowing firefighters to step up efforts to bring the blaze under control.

The intensity of the blaze, fuelled by unseasonably hot weather and high winds, had alarmed firefighters.

The BBC quoted RFS Commissioner Shane Fitzsimmons as saying the fire was behaving "very aggressively". The strong winds have been pushing the flames north and east towards suburban areas, with fire crews using water-bombing planes to try to tame the fire front.

Prime Minister Malcolm Turnbull 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.

NSW Police have launched an investigation into the cause of the fire and are appealing for witnesses, ABC online reported.

Mr Fitzsimmons said the hot weather was sucking "the moisture out of the vegetation", making it highly susceptible to catching fire.

"It will be a tough week for firefighters because unfortunately what we really need is a meaningful rain right across this fire ground and right across other parts of NSW," he said.

More than 200 people visited an emergency centre at Moorebank Sports Club overnight, with local hotels offering accommodation to residents unable to return to their homes, ABC said.

Liverpool Council chief executive Kirsten Fishburn said locals had come together to help.

"I've seen people offer up their houses, their beds, to babysit kids, to look after injured reptiles. Anything the community needs, people have offered up their hands to deliver it," she said.

Local businesses have also donated food and water to firefighters, ABC said.