Crews complete undersea wastewater pipeline to boost New Zealand infrastructure
(UI) — A 19.5-ton tunnel boring machine (TBM) returned to Onehunga Wharf after completing a $22 million outfall tunnel beneath the Waiuku Channel, part of an infrastructure upgrade for Clarks Beach.
Over the past seven weeks, the 1.2-meter-wide (about 4 feet) TBM tunneled a 240-meter (787 feet) path under the seabed, reaching depths of around 13 meters (43 feet). The effort supports ongoing upgrades to the Clarks Beach Wastewater Treatment Plant and prepares the region for population growth projected to reach 30,000 by 2050.
“The outfall has been designed and sized to support future population growth,” said Dave Kennerley, program delivery manager. “It will carry highly treated wastewater roughly 100 meters (328 feet) into the Waiuku Channel—a dynamic part of the harbor where it will quickly disperse.”
Kennerley emphasized that only high-quality treated water will be discharged, and only during outgoing tides to minimize impact.
Jason Salmon, project manager, said the TBM emerged into a receiving pit 10 meters (33 feet) below the water’s surface last month. It then remained at the seabed for two weeks while divers sealed the tunnel to prevent seawater intrusion and secured the TBM to a floating barge.
“After a seamless drive, the TBM is now undergoing maintenance as we prepare for the next stage,” Salmon said.
Crews are now focused on installing the 66-meter (217-foot) diffuser that will connect to the tunnel. This includes driving seven large columns into the seabed using a support vessel called Tūhura. The diffuser will release treated water through 22 duckbill-shaped nozzles designed to prevent seawater from backflowing into the system.
The new outfall will go into service alongside the upgraded wastewater treatment plant, expected to be operational by June 2026.
Residents and visitors can watch construction continue from nearby spots such as Waiau Beach, Clarks Beach Golf Club, and the Clarks Beach Yacht Club until project completion in late 2025.
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