More than 500 homes saved in Sydney blaze as residents warned to stay vigilant

As firefighters continue battling a Sydney bushfire that may have been deliberately lit, it's been revealed more than 500 homes were saved over three days.

Residents and firefighters have saved 500 homes from a ferocious bushfire in southwest Sydney. Source: AAP

More than 500 homes have been saved by Sydney firefighters and residents during the course of a fierce bushfire that's blackened about 2800 hectares over three days.

Crews continue to monitor the fire burning southwest of the city after its status was downgraded from out-of-control to being controlled.

NSW Police are treating the blaze, which began at Casula on Saturday, as suspicious and have set up Strikeforce Carpi to investigate.

Residents and firefighters have saved 500 homes from a ferocious bushfire in southwest Sydney. (AAP)

The NSW Rural Fire Service revealed 528 homes, three facilities and one outbuilding were saved after an assessment team gained access to the blackened fire zone.

Two homes were damaged and one cubby house was destroyed but RFS Commissioner Shane Fitzsimmons says it's "extraordinary" there were no major losses.

Mr Fitzsimmons on Monday estimated a large back-burning operation may lead to the fire being contained in the next day, but warned it may still be some time before the affected area is deemed safe.

If the fire was deliberately lit, those responsible could face a punishment of up to 25 years behind bars.