Huge Sydney bushfire suspected to be a 'sickening and reprehensible' act of arson

If a huge bushfire in Sydney's southwest on the weekend was deliberately lit, as authorities suspect, it's a "sickening and reprehensible" act of arson, the NSW Rural Fire Service commissioner says.

Fire investigators are working with NSW Police's Strikeforce Carpi to determine the cause of the blaze which started on Saturday at Casula and is being treated as suspicious.

Detectives want to determine if there is a link to several spot fires in the Leacock Reserve area on Friday night.

Less than 24 hours later a bushfire flared up about 2.30pm on Saturday in the same area.

It's now burnt through more than 2400 hectares of land and come dangerously close to homes.

"The area where we believe the fire commenced has been forensically examined," Acting Superintendent Paul Albury told reporters on Monday.

"We have taken a number of statements from people who called the police and fire services in regards to the fire."

Detectives have also looked at CCTV footage from around Leacock Reserve.

It's unclear if Friday's fires reignited on Saturday or if someone may have deliberately lit the blaze which has now burned through almost 2500 hectares of bushland focused on the Holsworthy military range.

RFS Commissioner Shane Fitzsimmons slammed the alleged firebug, saying any such act would be "sickening and reprehensible":

"How dare anyone in society think they can deliberately light a fire, endanger all these men and women - the vast majority who are volunteers - putting their lives on the line to try and bring these fires under control and, in turn, put so many members of the community (at risk)," the commissioner said.

Mr Fitzsimmons noted it was potentially "a very serious crime" with NSW having the toughest penalties in Australia.

Offenders can face hefty fines and up to 25 years in jail, he said on Monday.