More than 200 firefighters continue to battle a volatile and fast-moving blaze in Sydney's southwest but the NSW Rural Fire Service says there have been no reports of damage to homes so far.
An RFS spokesman said emergency services reported flames up to two storeys high and firefighters would continue to work through the night.
"It remains a really volatile and fast- moving situation," he told AAP late on Saturday night.
"It's being fanned by some fairly strong winds still."
The fire is moving southeasterly and has crossed Heathcote Road in Heathcote, moving towards Alfords Point, Menai, and Barden Ridge.
The RFS has told residents in this area it is too late to leave, advising them to seek shelter as the fire front approaches and to protect themselves from the heat of the blaze.
"Crews are recording five-metre flame heights which are pretty substantial at any time of the year," the spokesman said.
"It's a fast-moving and dangerous fire, and people need to be taking appropriate action to keep themselves and their families safe."
There are no reports of any structures being damaged, he added.
Hundreds of people have been evacuated from their homes, with the fire close to houses in Holsworthy and Wattle Grove, and spot fires moving towards Sutherland and Menai.
Residents from Holsworthy and the Holsworthy military base were told at about 10.30pm they could return, with roads reopened.
However, roads are not open to Voyager Point, Pleasure Point and Sandy Point, with residents who stayed put advised to shelter in place and not to leave their homes, and those who left unable to return overnight.
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."
An RFS emergency warning was first issued after 3.30pm on Saturday for the fast-moving bushfire, with residents in Wattle Grove and the Holsworthy military complex told to shelter in place.
A couple of hundred people are taking shelter at Moorebank Sports Complex, including many young families and some people with pets, all eagerly waiting for updates from police.
Diana Helmich evacuated her Pleasure Point home with her husband and two dogs at 4.30pm after receiving a text message alert.
"You could see the smoke - it was really, really black in the backyard," she said from Moorebank Sports Complex.
"It's a waiting game at the moment ... hopefully they'll get it under control."
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.
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.