Another worker was in critical condition while two others were in serious condition.
Four workers were found trapped inside an eight-metre-deep manhole at the plant operated by the Drainage Services Department on Thursday afternoon.
Firefighters and ambulances arrived at the scene to launch a rescue operation.
The fire service said officers faced severe difficulties, including the fact the hole’s entrance measured only one square metre, the darkness and the danger of a rapid rise in the level of the water.
The workers were later rescued by firefighters, but a female employee later died in hospital.
Her three male colleagues were also taken to hospital.
The Labour Department has launched an investigation into the cause of the incident.
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Last updated: 2021-11-18 HKT 23:00