A plane dousing wildfires in bushland south of Sydney has crashed into a national park, killing the pilot and sparking a new fire to add to 55 burning across New South Wales state in Australia.

The accident happened as more dry and windy conditions caused a flare-up in huge fires burning for a week in mountains to the west of Sydney.

"It's hard to definitely say that [the worst is over] at this stage," Rural Fire Service spokeswoman Natalie Sanders said.

Emergency officials confirmed a 43-year-old man died when his water bomber fixed-wing aircraft crashed in the Budawang National Park.

Rescue workers were unable to retrieve the body, driven back by high winds, smoke and the rugged terrain.

"It's a tragedy for the firefighting community," said RFS Commissioner Shane Fitzsimmons.

More than 200 homes have been destroyed in New South Wales since last Thursday, when fires tore through Sydney's outskirts.

The fires have burned through tens of thousands of acres and at one stage had a combined perimeter of about 1,600km.

One of the largest blazes was started by a live firing exercise on army land, the fire service said, prompting an apology from the acting defence chief.

Police have arrested several children suspected of starting some fires, while others were sparked by lightning or power lines arcing in strong winds, according to the fire service.

The state government has declared an emergency enabling it to order evacuations, hoping to avoid a repeat of the 2009 "Black Saturday" fires in Victoria state that killed 173 people.