The men, aged in their 60s, remained in serious but stable condition after being transported to two separate hospitals, Ambulance NSW said on Monday.
One of them sustained a leg injury and the other injured his neck, a spokesperson for the emergency service told AAP.
The roof and wall of the building on Wattle St in the Sydney suburb of Ultimo caved in about 11am on Monday, causing debris to fall onto the street and a neighbouring Shell petrol station.
Piles of large bricks were strewn on top of each other and the destroyed roof damaged vehicles parked nearby.
Another five people were assessed by ambulance personnel and released without further treatment.
"A portion of that building has collapsed, and some of the debris has come down and fallen on some cars parked within the service station area," Fire and Rescue NSW Acting Superintendent Peter Murray said.
The building was still under construction.
One patient was transported to St Vincent's Hospital and the other to Royal Prince Alfred Hospital.
One of the men was initially misidentified by authorities as being in his 30s, with his age later confirmed to be in his 60s.
Urban search-and-rescue firefighters are at the scene, with police clearing the area and evacuating workers from the construction site.
Surrounding buildings were also evacuated and police urged the public to stay away from the area.