Labour Secretary Chris Sun on Wednesday said initial investigations suggest legal requirements for underground work were not followed at a site where two employees died at the weekend.
The pair had gone to change pipelines near the Elements mall in West Kowloon on Saturday. Their bodies were found a day later after they apparently succumbed to methane gas.
Sun told an RTHK programme that failing to take proper precautions before workers go into enclosed spaces is illegal.
"You have to get a qualified person to do a risk assessment in advance to ensure the space is suitable to work in, and then you have to write the information on the risk assessment and the workers involved on a form and display it somewhere noticeable," he said.
"According to our preliminary investigation, we haven't found evidence showing a risk assessment was sufficiently done, and we don't see the notice that must be put up."
Sun said authorities will take legal action against all contractors and subcontractors in case of violations.
He added that officials will consider ways to better ensure the safety of workers, including having contractors notify the Labour Department every time workers go into enclosed spaces.
Separately, the Development Bureau said it has banned the contractor involved in the weekend's incident from bidding for public projects.
The bureau said the firm would have to review its safety management system and carry out improvement measures before it would consider lifting the ban.