Two people have been taken to hospital in a serious condition after a building collapse in Sydney that sent debris falling onto vehicles at an adjacent service station.

A wall and roof of the building in Wattle Street, Ultimo, collapsed around 11am on Thursday, sparking chaotic scenes as emergency services converged on the site.

Ambulance NSW said a man aged in his 30s and another in his 60s were rescued from the rubble and taken to separate hospitals in a serious condition, while five others were assessed by paramedics.

The site is next to a service station, and Ten News reported that around 35 people were evacuated from the immediate vicinity while up to 100 more were evacuated from a red-brick building behind it.

Fire and Rescue Acting Superintendent Peter Murray said one the injured men was a customer at the service station who was getting back into their car when the incident occurred, while another had been sitting in their vehicle.

“As a precaution, the service station behind me has been shut down,” Murray told reporters at the scene.

“It was chaotic at first – these type of things naturally are… our first priority is the life safety of anybody that’s involved.”

Murray said specialist urban search and rescue firefighters and drones were used to assess the safety of the building, but there were no reports of anyone being trapped or missing.

“Situations like this obviously are very dangerous, particularly with the bricks, the walls, the weight of some of those bricks coming down as well,” he added.

ABC News reported that the warehouse building previously housed a rehearsal studio but had been approved for demolition to make way for a multi-storey mixed-use development.