John O'Brien had left for work at 3am on Thursday, leaving his wife Bianka and one-year-old son Jude asleep in their apartment above a Rozelle convenience store.

In the flat across the hall was their neighbour, Chris Noble.

Mr O'Brien returned home after hearing a large explosion and fire had ripped through his street, fearing the worst. And within hours, police confirmed the trio were unaccounted for, as the building crumbled in a series of smaller explosions after the initial blast that rocked Sydney's inner west. Police now fear it was deliberately lit.

A rescue operation continued late on Thursday night. Shortly after 7pm, rescuers confirmed a male body had been found in the wreckage.

Hopes were fading that Ms O'Brien, 30, Jude, and Mr Noble, 30, would be found alive among the rubble of the destroyed premises on Darling Street.

Two other residents jumped from the first storey to escape the blaze, while the convenience store owner, Adeel Khan, became trapped underneath a fridge and other rubble when shop's roof collapsed shortly after the bang. Only his hands could be seen when rescue crews arrived.

Fire crews who heard and felt the explosion up to a kilometre away at Balmain managed to free Mr Khan before he and the two men who had jumped to safety were taken to Royal Prince Alfred hospital.

He remained in intensive care on Thursday night.

Initial attempts to search for Ms O'Brien, Jude and Mr Noble were hampered by fears the remaining structure could collapse on rescuers. Late Thursday crews were combing through the debris brick-by-brick but more than half of the rubble remained unsearched by nightfall.

One firefighter suffered minor facial injuries which searching.

''We are still trying to find somebody but it's a dire situation that they find themselves in,'' NSW Fire and Rescue Superintendent Paul Johnstone said.

Council workers collecting rubbish on Thursday morning have told police they saw a car drive past the two stores and then do a hasty U-turn in the moments before the blast.

Police said they had not had problems at either store before, but that they were treating the blaze as suspicious.

Officers spent Thursday searching nearby streets and lanes for clues.

Inspector Clive Ainley said there was ''plenty'' of CCTV footage, and police were in the process of gathering and examining it to determine what had happened.

Anthony Carroll, who lived with his father in a unit above the convenience store, said he was fast asleep when he was woken by the explosion.

Mr Carroll said he turned on his bedroom light and saw smoke coming in under the door.

''Within 30 seconds, my room was full of smoke,'' he said.

''I just grabbed what I've got on and got out of there. There were people screaming and yelling and people trapped in the place next door so it was pandemonium. No one knew what was going on.''

The investigation is being led by the NSW State Crime Command's Arson Squad, with 12 officers examining the scene on Thursday.

They were assisted by members of the homicide squad and the local area command.

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