Safety measures for working in confined spaces also apply to sites connected to manholes, a construction group said on Wednesday, following the deaths this week of two sewer maintenance workers who were suspected to have been exposed to toxic gas in Sha Tin.

Authorities said the pair, who are thought to have inhaled hydrogen sulfide, were employed by a contractor to clean sewers using high-pressure water jets, but their work should not have required them to enter the manhole.

Lee Kwong-sing, a safety adviser with the Hong Kong Construction Industry Employees General Union, said people working near a manhole could also be exposed to a high concentration of gas when the cover is opened.

"If the [hazardous] gas is concentrated enough, people near the manhole may have a chance of inhaling a high concentration of toxic gas... and possibly faint and fall into the manhole," Lee told an RTHK programme.

He also said current regulations stipulate that precautions are required for people working in a confined space and also in the vicinity of the site.

"In this case, workers were using high-pressure water jets near the manhole," Lee said. "So one can't say because workers are not going into the manhole, there is no need for precautionary measures."

On RTHK's Hong Kong Today programme, unionist lawmaker Lam Chun-sing said the government needs to tighten up guidelines for people working in confined spaces, including more stringent tests on hazardous gases and mandatory video monitoring of workers inside manholes.

Labour and Welfare Secretary Chris Sun on Tuesday evening said a revised code on working in confined spaces would be ready in one to two months.

Lam, who chairs the Federation of Hong Kong and Kowloon Labour Unions, agreed with suggestions that construction companies should notify authorities before work starts.

"For those small-scale construction works, related to high-risk tasks such as [working in] confined spaces and manholes, you also need to report and notify the Labour Department," he said.

Currently, contractors are not required to submit project reports to labour officials in advance if such work will involve at most 10 workers and will be completed within six weeks.