The original NZ$1.2bn (£500m) budget to build a main tunnel plus two links sewers, 16 shafts and associated infrastructure was drawn up in 2018 and construction commenced in 2019. The cost rose to NZ$1.32bn when the scope of the project extended from Western Springs to Grey Lynn, connections added to service new developments in Mt Roskill, along with improving water quality on the western isthmus.
Unprecedented inflation since late 2021 has resulted in a 15.5% forecasted increase of NZ$204m to complete the works, bringing the total budget to NZ$1.523bn. The increase covers construction of the tunnel to Tawariki St, Grey Lynn (due for completion mid-2026) and includes provision of NZ$109m to address the impacts of ongoing inflation to the end of the project.
Central Interceptor executive programme director Shayne Cunis said construction was progressing across all 16 sites, with 72% of work completed.
“Hiwa-i-te-Rangi, our tunnel boring machine, has broken through to a shaft in May Rd, Mt Roskill and is more than halfway on her 14.7km journey to central Auckland. We’ve completed one link sewer and will finish the first drive for a second link sewer at the end of the month. Milestones are coming thick and fast,” he said.
“We absorbed the costs incurred through the Covid-19 pandemic and we had an allowance for inflation within the original budget, but the level of inflation that has been experienced, it became clear that these rising costs would exceed our budget allowance. Additional funding will come from within our resources and not Auckland Council and will allow us to complete the project as planned at the end of 2026.”
The Central Interceptor tunnel will connect with combined stormwater/wastewater networks in older suburbs such as Blockhouse Bay, Mt Roskill, Grey Lynn and Mt Albert, taking flows to Mangere Wastewater Treatment Plant for processing, resulting in an 80% reduction in wet-weather discharges into local waterways. It is the largest wastewater infrastructure project in New Zealand history.
Last year, resource consent was given to extend the main tunnel to Pt Erin to capture combined stormwater and wastewater flows from the Herne Bay/S Mary’s Bay neighbourhoods. The additional 1.5km Grey Lynn-Pt Erin extension, requiring an additional 12 weeks’ tunnelling, will create an overall new length of 16.2km and will be costed separately from the NZ$1.523bn.
