A fire which has been raging in Sydney's southwest, causing hundreds of people to evacuate their homes, may have been deliberately lit, NSW police say.
Scores of fire fighters are continuing to battle the blaze on Saturday evening, which is close to homes in Holsworthy and Wattle Grove, with spot fires now moving towards Sutherland and Menai.
Emergency services were first alerted to the blaze at about 2.45pm on Saturday, Inspector Mitch Newtown, duty officer at Liverpool police station said.
"We believe it might have been a deliberately lit fire," Insp Newtown told AAP.
"We want to turn our focus to the investigation once the welfare of the people is taken care of."
A NSW Rural Fire Service emergency warning was first issued after 3.30pm on Saturday for the fast-moving bushfire, with residents in the Wattle Grove community and Holsworthy military complex told to shelter in place.
A few hundred people are taking shelter at Moorebank Sports Complex, including many young families and some people with pets, eagerly waiting for updates from police.
Insp Newtown said police were called to a few spot fires at about 10pm on Friday in Casula, which may have also been deliberately lit.
"It's a possibility today's fire has been some of those spot fires reignited," he said.
Police are hoping residents can return home on Saturday night, but emergency accommodation forms are also being distributed at the evacuation centre.
Residents in Voyager Point, Pleasure Point and Sandy Point are now also being advised to shelter in place and not to leave their homes.
Heathcote Road has been closed and buses are replacing trains between East Hills and Glenfied on the T8 airport and south line, with the Transport Management Centre telling people to allow plenty of additional travel time and to check indicator boards.
There are no reports of homes being damaged by the blaze, a NSW RFS spokesman said.