The blast on Waikanda Crescent at Whalan in Sydney's west happened just before 1pm on Saturday, blowing out windows and damaging neighbouring townhouses in the complex, Fire and Rescue NSW said.
Firefighters and others at the scene rescued two people - a woman in her 60s and a wheelchair-bound woman in her 70s - from the ruins.
However, they were still searching about 3pm on Saturday to determine whether other people were trapped in the rubble.
A NSW Ambulance spokesman said paramedics had so far transported five people to hospital - three to Mount Druitt Hospital and two to Hawkesbury District Health Service.
"The situation is currently still unfolding," an ambulance spokesman said.
A gas company was on site on Saturday afternoon trying to isolate the mains after it was unable to cut off supply.
"There is no fire at present but the scene remains volatile and unstable," Fire and Rescue NSW said.
The fire service had more than a dozen units at the location and was also using drones to assess the blast zone.
Firefighters urged the community to stay away from the scene, while NSW Police from the Mount Druitt area were also attending.
A NSW Police Force spokeswoman said an investigation into the circumstances surrounding the explosion had begun.
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