“Sadly, a serious incident has happened again before the revised code has been implemented,” the Association for the Rights of Industrial Accident Victims said on Tuesday.

“We are very angry about the frequent occurrence of fatal incidents in confined spaces.”

The two men, aged 26 and 34, were found unconscious inside a four-metre-deep (13 feet) manhole. The pair were cleaning drains at Yuen Wo Playground in Sha Tin with two other colleagues.

Another colleague alerted police just before midnight that the pair had lost consciousness underground. Firefighters rescued the pair, who were taken to Prince of Wales Hospital in Sha Tin but certified dead soon after arrival.

Police said the cause of death would be determined by a postmortem examination. Initial investigations suggested the pair had passed out after inhaling the gas.

The Labour Department said it was carrying out an on-site investigation.

“We will complete the investigation as soon as possible to identify the cause of the accident, ascertain the liability of the duty holders and recommend improvement measures,” a department spokesman said.

“We will take actions pursuant to the law if there is any violation of the work safety legislation.”

Development minister Bernadette Linn Hon-ho told lawmakers on Tuesday that workers did not need to enter the sewers under standard procedures, and were only required to use a high-pressure hosepipe to clean the drains from outside.

She said a key focus of the investigation would why the men had gone underground.

The 34-year-old victim’s father also questioned why the pair were working in the manhole. “Why did they let people go in?” he said.

The contractor had offered to provide the family with HK$500,000 in aid, he told local media, adding it was not enough to cover their daily life.

Labour authorities have been revising the code of practice for safety and health at work in confined spaces after two workers were killed last September by a suspected biogas leak underground at a construction site managed by railway operator the MTR Corporation.

Secretary for Labour and Welfare Chris Sun Yuk-han said on Tuesday evening that his bureau and police had joined hands to investigate the accident.

He said the revised code would be ready for release in one to two months and would include more technological means to enhance work safety inside manholes.

The association said that before working in a manhole, a rigorous risk assessment should be carried out to ensure no unknown gases were inside.

It urged workers to wear safety belts and life ropes for a timely escape, and called on the Drainage Services Department to ensure contractors complied with guidelines and regulations.

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The association also urged the government to make it compulsory under the revised code for contractors to submit project reports to the Labour Department in advance, including work content, construction dates and safety plans, to ensure authorities were informed and could conduct inspections.

A union also accused the government of failing to plug safety loopholes over the years despite a similar case in the same area almost two decades ago.

“The Drainage Services Department has not learned the painful lesson and tragedy has happened again 18 years later,” the Hong Kong Construction Industry Employees General Union said.

Two workers, both aged 46, died in a sewer manhole, also on Yuen Wo Road in Sha Tin on August 1, 2006.

Investigators suspected one man inhaled methane gas and passed out and his colleague then went into the manhole to rescue him but also fell unconscious.

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A Drainage Services Department inquiry found that the main contractor had subcontracted work and conducted work outside the intended area without permission, and failed to take necessary precautions to protect the workers.

“The Labour Department must seriously learn the hard lessons from various industrial accidents, review the loopholes in supervision and enforcement as soon as possible, make up for deficiencies and strengthen supervision and inspection,” the union said.

A string of industrial incidents have been reported in the past few years.

On December 22, 2021, two workers died after being pulled unconscious from a manhole at a construction site near Hong Kong airport.

The two men – Fan Chu-leung, 34, and Zhao Gao, 38 – were pronounced dead in hospital, while three colleagues who tried to rescue them complained of feeling unwell after inhaling suspected toxic sewage gases.

The five workers were sent to the site by their company to carry out cleaning work and inspect the manhole, according to a police source.

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The insider said an initial investigation suggested suspected sewage gas and water spilled into the manhole when two of them were climbing down. The pair then fell about 2.5 metres to the bottom of the manhole.

On November 18, 2021, a woman died and three men were seriously injured after falling into an eight-metre-deep manhole at a sewage treatment plant on Lantau Island.

Firefighters found the woman lying face down at the bottom of the manhole, in sewage water about 50cm deep, while the three men were trapped on a work platform six metres below ground level.

On January 7, 2020, three workers were suspected of inhaling an unknown gas while working in a manhole in Tuen Mun. One was rescued by firefighters after falling into a coma before later regaining consciousness.