Sydney fire video referred to police

Jamie McKinnell and Julia Carlisle

A social media video showing three men laughing near flames in Sydney has been referred to police as they investigate the cause of a massive bushfire in the city's southwest.

Revelations of the footage, which appeared on Snapchat and was forwarded to Bankstown officers on Tuesday morning, came as the NSW Rural Fire Service finally declared the blaze was under control.

It shows the trio dressed in black, one wearing a hat and another a bandana.

"Police would like to speak to the people in the video to assist with their inquiries," a spokesman told AAP.

It's understood the men aren't suspected of being responsible for the fire but may be able to shed more light on its origins.

Cooler conditions on Tuesday helped firefighters establish containment lines around the huge blaze and it was considered under control by 7pm.

"Firefighters will remain on scene overnight patrolling and consolidating containment lines around the fire," the RFS advice stated.

The fire, which started on Saturday and blackened nearly 4000 hectares, is burning within the Holsworthy range and around Barden Ridge.

Tuesday's favourable weather conditions were significantly cooler than the weekend when the fire was also fanned by strong winds.

RFS spokesman Matt Sun told AAP there were southerly breezes about 20km/h with humidity hovering around 60 per cent.

More than 200 firefighters were involved in the operation on Tuesday and the back-burning generated a significant amount of smoke that's drifting north across areas including the city and western Sydney.

Some roads were reopened including those to Pleasure Point and Voyager Point for residents with photo identification. Locals were urged to stay up to date by checking the RFS website.

More than 500 properties were saved, it was revealed on Monday night.

Two homes were damaged and one cubby house destroyed with RFS Commissioner Shane Fitzsimmons stating it was "extraordinary" there'd been no major losses.

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

If the fire was deliberately lit those responsible face up to 25 years behind bars.