An investigation has been launched after two sewer maintenance workers died in a manhole in Sha Tin.

Authorities say early indications are that the pair, and two other men who also fell ill while working in the manhole, inhaled hydrogen sulphide.

The Drainage Services Department said the men 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.

Firefighters were called at around midnight to reports that the workers had fallen ill while operating three to four metres underground at the Yuen Wo Playground.

The two men are said to be 26 and 34 years old. They were declared dead at the Prince of Wales Hospital. The two other workers were also treated at the same hospital.

The department said it had ordered a halt to work at the site and paused all similar work elsewhere until it could conduct safety checks.

It’s also launched an investigation and promised to work with the Labour Department and the police, who are also looking into the case. It's been classified as an industrial accident.