Rescue teams are working around the clock to painstakingly comb through the ruins of a collapsed townhouse, desperate to find a woman feared trapped alive.
An explosion on Waikanda Crescent at Whalan, in Sydney's west, levelled most of the two-storey home just before 1pm on Saturday, blowing out windows and damaging a neighbouring townhouse.
NSW Fire and Rescue Commissioner Jeremy Fewtrell on Sunday said crews remained hopeful of safely retrieving the woman feared trapped beneath.
"This is still well within the window for someone to survive," he told reporters.
"The bottom line is there's been the total destruction of a large building, and it's the job of the rescuers to work their way through that rubble to try to find spaces in that collapsed area where someone might have been caught."
Rescuers combing the area are specialists in structural collapses and the different ways that buildings fall under stress, giving them an indication of where to look for survivors, he added.
"You might recall in 1997, we had the Thredbo landslide ... it was almost a week when Stuart Diver was rescued, hence the effort to really be exploring the building as thoroughly as we can."
Dozens of search and rescue technicians, including firefighters who responded to last year's earthquake disaster in Turkey, have been working at the scene.
Firefighters are using acoustic monitoring devices to capture any sounds under the ruins while search cameras are being used to peer under the rubble and concrete slabs to look for possible survivors.
A police detection dog has been deployed over the debris, with no indications of human activity found so far.
Jackhammers and chainsaws are also being used to break up the rubble.
Mr Fewtrell said rescue teams were hampered by a series of setbacks while searching through the night.
"There's been a range of complications throughout the operation," he said.
"The job of rescue (teams) is to work their way through and try and find spaces in that collapse area where someone might have been caught.
"Part of that work involves this sort of very manual labour of piece by piece, picking up the debris and moving that away."
Complications included a gas leak, safety concerns about how much work could happen at the site and fire underneath the rubble in the collapsed area.
Firefighters have been unable to directly reach the fire because of the debris.
"But we are able to apply water onto that, the water obviously then drains down and suppresses the fire," he said.
Crews rescued a woman in her 60s and a wheelchair-bound woman in her 70s from the ruins on Saturday afternoon, and later saved a small dog that was unharmed.
The women were among five people at the scene taken to hospital - three to Mount Druitt Hospital and two to Hawkesbury District Health Service.
All had been released by Sunday morning as emergency crews continued to search the building for any signs of life in the rubble.
The missing person was on the second floor of the building when it collapsed.
Emergency services are yet to establish what caused the blast.
NSW Police, who said the incident was under investigation, were initially called to reports of a gas explosion.
Gas mains were isolated at the property earlier on Saturday so crews could safely step up their search and use concrete cutters to remove the debris.
