Two workers plunged to their deaths on Monday after the gondola platform they were on fell from a building on Conduit Road in Mid-Levels.

The workers, whom police said were middle-aged, were about 20 storeys up at Realty Gardens when the accident happened at about 11:30am.

The Labour Department said it had sent staff to the site to carry out an investigation.

Siu Sin-man, chief executive of the Association for the Rights of Industrial Accident Victims, said the workers were inspecting the cradle ahead of planned external wall maintenance when the accident happened.

She said her initial understanding was that the platform's cables snapped during the inspection.

Siu questioned why the inspection had taken place at such a height, saying it could easily have been done just a metre above ground.

"They only need to test if it can move up and down and any problems with the parts. There's no need to go up so high. The Labour Department needs to investigate and the employer needs to give an account to the victims' families," she said.

Siu added that the two workers had around five or six years of experience in their roles.