Secretary for the environment KS Wong said at the breakthrough ceremony that the project demonstrates the government's commitment to upgrading Victoria Harbour’s water quality.
The Harbour Area Treatment Scheme began in 1994 to improve the harbour’s water quality by collecting and treating sewage generated from both sides of the harbour.
At a total cost of over HK$20bn (£1.6bn), it is the largest sewerage infrastructure project in Hong Kong.
Stage 2A works began in July 2009, and include a 21-km sewage tunnel at depths from around 70 metres down to 160m below sea level – deeper than a 50-storey building.
Breakthrough of the 3.2km section between North Point and Wan Chai was completed on 26 October.
Sewage from eight preliminary treatment works to Stonecutters Island for further treatment.