Three injured in scaffolding collapse at Sydney building site7News camera man Paul WalkerThree injured in scaffolding collapse at Sydney building site

Two construction workers were taken to hospital and a bystander was treated for shock after 12 storeys of scaffolding collapsed on a Mascot building site in Sydney.

The incident occurred at lunchtime and the men were at the top of the steel frame when it suddenly gave way.

The two workers rode the structure from the top, all the way to the bottom.

Emergency services were called to the 12-storey building in the city's south-east, just around 1pm this afternoon.

Around 100 people were evacuated from the building site after the scaffolding gave way.

One man has limb and chest injuries, the other escaped with cuts and abrasions.

Workers have told 7News there were few people on site because it was lunchtime, and that the scaffolding was being dismantled at the time of the collapse.

The cause of the accident is not known but a major Workcover investigation is under way.

A CFMEU official says they visited the site just before Christmas in relation to serious concerns about the scaffolding.

Construction had to be shut down for a day while rectification work was carried out.

Power lines were brought down and there were concerns fuel was leaking from vehicles stuck under the collapse.

O'Riordan Street was closed in both directions but has since reopened.

"Motorists travelling to the airport are advised to allow plenty of extra travel time," the Transport Management Centre said.