The bushfire threat is easing in Sydney's west after a day of high temperatures and strong winds fanned four major blazes.

All four emergency warnings have been downgraded at Castlereagh, Marsden Park, Windsor and Winmalee.

About 800 firefighters were deployed today to battle the fires that destroyed two houses at Marsden Park and Winmalee, as well as cars, sheds and other property.

Seven firefighters were injured, suffering smoke inhalation and minor burns.

The Winmalee fire, which has now been downgraded to watch and act, crossed Hawkesbury Road this afternoon and was heading toward the Yellow Rock area.

Many residents voluntarily left their homes, and authorities warned of a tough night ahead as firefighters worked to protect properties at Hawkesbury Heights and Yellow Rock.

The bushfire season may have made an early September appearance as dangerous fire conditions extend across the state, where 40 other fires are also burning.

The lack of rain, winds and temperatures rising past 30 degrees have made conditions difficult for firefighters.

But RFS Assistant Commissioner Steve York says conditions this evening have eased, helping firefighters get a handle on the blazes.

"With the easing weather conditions over this evening and hopefully tomorrow, that will give firefighters an opportunity to do a lot of containment work that we need to do to get these fires under control," he said.

Bureau of Meteorology forecaster Simon Lewis says the temperatures have fallen across the Sydney basin.

He says winds are also dropping and shifting around to the south-west, which should bring relief to firefighters.

Assistant Commissioner York would not be draw on claims that the Winmalee fire may have been a hazard reduction burn that got out of hand.

"Our fire investigators will be on the fire grounds tomorrow again looking at all the fires that were in and around the Sydney basin and making an assessment of how those fires were started and so on," he said.

"So that assessment will be made in the coming days."

State Police and Emergency Services Minister Mike Gallacher says the penalties for anyone caught lighting fires deliberately will be severe.

"Three [fires] are on a particular part of north-western Sydney, and there's a question about those fires, and they'll be investigated tomorrow," he said.

"There's the one at Winmalee up into the foothills of the mountains. They suspect that may well be a different situation.

"We won't know, of course, and the experts won't know until they can get in there to have a bit of a look."