Firefighters battling bushfire praised for their hard work

Firefighters and volunteers have been praised for their hard work in battling horrific bushfire conditions in Sydney's south west.

On Sunday, the NSW Rural Fire Service shared a picture online of a firefighter cradling a rescued koala in a spare shirt.

In the post, the RFS commended the work of all firies - but warned there was still "a lot of very hard work" to be done.

Dozens of people have responded to the Tweet, many saying they are truly grateful for the effort of those emergency services personnel involved.

"Thanks for keeping people, property and animals safe," one person wrote.

"We appreciate your hard and brave work," another said.

The bushfire that burned through the Holsworthy military range has impacted houses in Menai and Barden Ridge, the NSW Rural Fire Service has confirmed.

Strong winds pushed the flames north and east towards suburban streets on Sunday afternoon as 15 firefighting and reconnaissance aircraft joined the operation.

At 4.30pm, RFS spokesman Greg Allan confirmed some houses had been impacted.

"What we don't have information on is the type of damage and number of properties," he told AAP.

Residents have been warned of the risk of ember attacks as more than 500 firefighters from the Rural Fire Service, Fire and Rescue NSW and the Australian Defence Force continued the mammoth operation to keep things under control.

Prime Minister Malcolm Turnbull visited the RFS command centre in Sutherland in the afternoon.

"Clearly with the wind increasing in strength the situation is worsening but there are outstanding efforts to control this fire being undertaken," the prime minister told reporters.

"This huge blaze has been fought in very high temperatures and very high winds - unseasonal conditions."

More than 70 trucks are lining the roads dividing bushland from residential homes while water-bombing helicopters target the blaze from above.

The fire - which has burnt 1000 hectares - was upgraded on Sunday at 1pm when the RFS issued a fresh emergency warning.

"We are seeing the fire behave very aggressively - it's spreading to the east and to the north," RFS Commissioner Shane Fitzsimmons told reporters in Heathcote.

"The overriding message is we still have a very volatile, dynamic and dangerous fire situation."

Residents in Voyager Point, Pleasure Point, Sandy Point and Illawong, Menai and Bangor, to the east of Alfords Point Road, have been told to expect ember attacks.

"Residents should shelter in place, it is too late to leave," the RFS warning states.

Residents in Alfords Point and Barden Ridge, to the west of Alfords Point Road, have also been told to shelter in place as the fire approaches.

The fire tore through the highly secure Holsworthy military range on Saturday afternoon and was upgraded to an emergency level when it jumped Heathcote Road.

It was downgraded at 6am on Sunday but the situation worsened in the early afternoon. While there are strong winds of up to 60km/h, the temperature is lower than Saturday and there's increased humidity both of which helps firefighting efforts.

"(But) we could see a deterioration in impact and damage this afternoon given the ferocity of these fires," Mr Fitzsimmon said.

Sunday afternoon is the critical period with the high winds not expected to ease until "late into the evening".

Fire investigators and police are working to establish how the fire was started. NSW Police have declared the area a crime scene but the RFS says it's too early to declare if the blaze was deliberately lit.