Fire investigators were to examine the ruins of the Macedonian Orthodox Church of the Resurrection in Rockdale early on Monday morning to determine the cause of the blaze, which broke out about 9.45pm on Sunday.

Superintendent Adam Dewberry, from Fire and Rescue NSW, said about 60 firefighters battled the blaze at the church on Frederick Street for several hours, before bringing it under control.

Large flames could be seen shooting from the roof of the church, which earlier on Sunday had hosted services for Orthodox Easter.

Churchgoers watched from the street as their place of worship went up in flames.

"[It's] Orthodox Easter at the moment, and who knows, maybe candles left? Never know," one church member said.

Superintendent Dewberry said about 60 residents were evacuated from the neighbouring nursing home, with fears the blaze could spread to that building.

"The fire did escalate rather quickly in the first calls [to triple zero]," Superintendent Dewberry said.

"There was some heat damage done to the windows [of the nursing home] that adjoin the church, and some of the church walls did collapse."

The nursing home residents were allowed to return to their rooms about 1am, he said.

Fire and Rescue NSW Commissioner Greg Mullins said three firefighters were struck by falling bricks when sections of the church walls collapsed. Two were taken to hospital in a stable condition, and were expected to make a full recovery.

"One of them, it hit him on the air cylinder that he was wearing, knocked him to the ground. He was dragged out by colleagues," Commissioner Mullins said.

"Another firefighter was struck by bricks along one side, on the shoulder, chest and leg."

Police set up a 100-metre exclusion zone around the church during the fire and traffic was diverted into surrounding streets. Frederick Street remained closed to west-bound traffic between Watkin Street and Linwood Avenue on Monday morning.

Victoria's oldest Greek Orthodox church, The Annunciation of Our Lady, was also damaged by fire on Monday.

But NSW police said there was no known link between the fires.

St George Inspector Sean Daley said police would begin to investigate forensic evidence on Tuesday now that the danger of a collapsed roof has been averted.

Police have not learned of any threats against the church and have no received no camera footage or witness accounts of suspicious presences around it before the fire.

In a separate incident, firefighters were called to an apartment fire in Waterloo in Sydney's inner south early on Monday morning.

Police and fire crews arrived at the building on Wellington Street just after 2am to find the second-floor unit well alight.

Residents in the building were evacuated, and a 77-year-old man was taken to Royal Prince Alfred Hospital with smoke inhalation.

The cause of the fire has not been determined. Investigations into both fires are continuing.

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