Probe into huge Sydney fire continues

Dominica Sanda and Julia Carlisle

Four men seen on a Snapchat video laughing near flames have been released without charges after being questioned by police investigating the cause of a huge Sydney bushfire.

The footage was forwarded to Bankstown officers on Tuesday morning and on Wednesday the men aged 18, 21 and two 20-year-olds spoke to police about the video, a NSW Police spokeswoman told AAP on Wednesday.

The men were released with no charges and were disregarded from any further investigations after detectives discovered the video was created on Monday evening in the Barden Ridge area, the spokeswoman said.

Investigations are continuing into the blaze which burnt nearly 4000 hectares in the city's southwest and is being treated as suspicious.

Firefighters finally got the upper hand on the fire on Tuesday night, declaring it under control more than four days after it began.

Smoke will continue to affect the greater Sydney area and the Blue Mountains on Wednesday following days of backburning operations.

The blaze, which may have been deliberately lit, is now at an advice level, the NSW Rural Fire Service said on Wednesday.

Firefighters will remain on site over the coming days to continue to mop up the area.

The fire began at Casula on Saturday afternoon and is the subject of an investigation by Strikeforce Carpi.

The blaze blackened nearly 4000 hectares near the Holsworthy range and Barden Ridge by the time it was contained by a back-burning operation.

More than 500 homes have been saved by the enormous firefighting effort.