Orongorongo Tunnel Wainuiomata Entrance – © Jeremy Foster
The Orongorongo Tunnel, also known as Semple’s Tunnel, is a significant infrastructure project in New Zealand’s water supply history. Constructed between 1921 and 1926, this 3.2-kilometre tunnel was designed to connect the Orongorongo River and valley to the Wainuiomata River and valley. Its primary purpose was to augment Wellington’s water supply by diverting water from the Orongorongo catchment into the Wainuiomata system.
The construction process faced considerable challenges, particularly in terms of accessibility. A 6.5-kilometre pack track ascending to 300 metres into the Orongorongo Valley was established to transport lighter materials. Heavier materials were transported by trucks from the mouth of the Orongorongo Stream over a distance of 45 kilometres. From there, horse teams hauled the materials an additional 22 kilometres up the riverbed to reach the tunnel entrance. This arduous route facilitated the transportation of approximately 600 tons of steel water pipes, cement, timber, and compressing plant equipment.
Upon its completion in 1926, the tunnel significantly enhanced Wellington’s water supply capacity, providing an additional 27 million litres per day. Remarkably, this system remains operational today, contributing around 5% of the Wellington region’s water supply.
The tunnel also includes a small-gauge railway line, which allows staff to service the pipeline and access the upstream catchment. Regular inspections are conducted to assess and mitigate geotechnical risks, ensuring the safety and functionality of this vital water supply infrastructure.
