The women were among five people at the scene taken to hospital on Saturday afternoon - three to Mount Druitt Hospital and two to Hawkesbury District Health Service.

All were in a stable condition with minor injuries on Saturday evening as emergency crews continued to search the building for another person feared trapped in the rubble.

"We know that it's the back side of the building where the most rubble has collapsed and that is the generalised area where we're searching now," Fire and Rescue NSW Superintendent Ross Genders told reporters on Saturday.

The missing person was on the second floor of the building when it collapsed, he said.

"At this time, all we know is that they're still alive," he said.

"We have heard taps and noises and that is generalised as being a good indicator that there's still life underneath there."

Firefighters were using seismic monitoring equipment to capture any sounds under the ruins. 

Supt Genders said the search and rescue operation would be protracted as crews removed the rubble piece by piece, hampered by wet weather.

Fire and Rescue NSW said late on Saturday the search would continue overnight.

Emergency services were 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. 

NSW Ambulance Chief Superintendent Ian Johns said the five people taken to hospital after the collapse had made a lucky escape.

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