Workers have been evacuated from a roadway tunnel project in Sydney’s south after a sinkhole caused a nearby building to partially collapse.

A multi-agency operation is under way at Rockdale, where crews are working to determine the extent of the instability, which has so far caused a two-level office block to sag in the middle. 

About 20 people were evacuated from the area but no one was inside the building at the time. 

Firefighters have set up laser-monitoring equipment to help detect possible further movement, while engineers are determining how to shore up the building.

Experts are also working to assess the extent and cause of the collapse.

Fire and Rescue NSW crews have established an exclusion zone and people have been asked to avoid the area. 

Authorities are at the scene of the sinkhole.

The cause of the sinkhole is unknown and workers in the M6 tunnel project below will not return underground until it is deemed safe. 

Premier Chris Minns said there had been an engineering challenge with the tunnelling works and experts were on the scene.

“I know the builders of the road regarded that part as one of the most tricky because it’s the closest to the surface at only 16 metres, whereas a lot of the rest of the project could be up to 60 metres underground,” he told reporters.

Roads Minister John Graham said the first priority for crews would be to secure the site and then determine the cause.

At the time of the collapse, excavation work was happening inside the tunnel, the Australian Workers Union confirmed to the ABC. The work has since ceased.

Steve Ackerman, a spokesperson for the union said a “small trickle of material” coming out of the surface of the tunnel and the decision to cease work came as it got “progressively worse”.

“The night shift were working they had all been removed from the tunnel, and the day shift were not sent down there to commence their shift,” he said.