A scaffolding collapse at a Kai Tak building site on Tuesday that left two workers dead could never have happened if the structure had remained correctly installed, according to a union safety adviser.

A worker was on the scaffolding when it plunged from the 19th storey of a housing block under construction, hitting four other people. Two of those struck by the scaffolding died, while the three other workers caught up in the accident were injured.

Lee Kwong-sing, a consultant with the Construction Industry Employees General Union, said scaffolding is normally attached to walls via poles called putlogs, and if these are solidly installed and then left in place, it's impossible for the scaffolding to collapse.

"I believe workers who put up the scaffolding had tied in enough putlogs properly, because if they didn't, the workers themselves may fall," he told an RTHK programme.

"But when others do subsequent procedures, say installing a glass wall, the putlogs could be in the way. So some workers may think, oh I will remove the safety devices temporarily."

Lee noted that by law, scaffolding needs to be inspected every two weeks, and that officials said the one that collapsed was checked last week. The consultant said the authorities need to probe whether the inspection was conducted properly, or if workers had removed putlogs afterwards.

Siu Sin-man, the chief executive of the Association for the Rights of Industrial Accident Victims, said on Commercial Radio that the husband of one of those killed in Tuesday's accident was so emotional that he had to be taken into hospital.

The couple's children were flying in from the mainland, Siu added.