Central Interceptor Project

Scope of works

The scope of work included:

• Construction of a 4.5m diameter interceptor tunnel extending 16.2km between 20 and 110m below ground, crossing beneath the Manukau Harbour at approximately 15m below the seabed, using tunnel boring machines.
• 18 permanent shafts for long-term operational access, including excavation, lining, and integration with the wider network system.
• Drop shafts, grit traps, flow control and overflow structures.
• Air vents and air treatment facilities.
• A pump station and rising main system supporting network integration and transfer to the treatment plant.

While the Central Interceptor is a major tunnelling achievement, a significant portion of the works focused on integrating and diverting existing wastewater networks into the new system, along with major upgrades at the Māngere Wastewater Treatment Plant.

These works included:

• Rising main sewer installation using open cut, battered trench, sheet piled trench, and trench shoring methods.
• Gravity sewer installation using tunnelling, micro tunnelling, pipe jacking, horizontal directional drilling, and open cut construction methods.
• Structural relining of existing sewer assets.
• Pump station retrofit and decommissioning at PS25 Miranda Reserve and PS23 Hillsborough.
• Sewer flow control chambers, including actuated gates, bifurcation chambers, and grit chambers.
• Biological air treatment facilities, including odour beds, biofilters, and overflow structures.

• In total, 100 network sewer work packages were delivered to enable system integration, staged commissioning, and future network connectivity.

The works also included a 5 cumec pump station, a rising main, and the reconstruction of the confluence chamber at Māngere, where the eastern, western, and south western interceptors converge before entering the treatment plant.

• Construction of a 5 cumec pump station and rising main.
• Reconstruction of the confluence chamber at Māngere.
• Replacement and relocation of treatment plant infrastructure, including odour beds, odour ducting, service lines, air, power, and recycled water systems.
• Integration of instrumentation, flow control gates, and plant operations through the Māngere Wastewater Treatment Plant SCADA system.
• Construction of the May Road air treatment facility, incorporating a biological odour control process.

Critical considerations and sensitive works

Delivery within the operational Māngere Wastewater Treatment Plant required extensive planning, analysis and vibration monitoring due to proximity to live critical infrastructure.

Works included isolation and upgrade of key treatment plant assets, including the complete rebuild of an odour bed and relocation of services, communications and water infrastructure, delivered in staged sequences to maintain continuous plant operation.

Safety and wellbeing

Worker safety and wellbeing were central throughout delivery. All personnel completed a two-day safety induction covering project risks and systems in place to support safe delivery.

A proactive safety culture was reinforced through recognition of high-performing teams and individuals. The team worked closely with Mates in Construction and WorkSafe New Zealand and, in partnership with Watercare, supported the ‘PPE For All’ programme, providing high-quality personal protective clothing and equipment to all workers.

Managing the programme and maintaining momentum

The project was delivered through significant challenges associated with the pandemic. Close collaboration with Watercare enabled delivery strategies to be adapted while maintaining programme momentum.

Key adjustments included reorganisation of work fronts, changes to subcontracting arrangements and shift patterns, additional resourcing for critical roles and workplace controls to reduce risk and disruption.

Stakeholder management and community engagement

Stakeholder management and community engagement were central to the delivery of the project, spanning 18 major construction sites between Māngere and Grey Lynn.

The team engaged with residents, businesses, schools and community groups throughout delivery through ‘Meet the Contractor’ events, community updates, newsletters, information packs, a 24-hour response line, school visits and support for local charities including Breast Cancer New Zealand. This was delivered in partnership with Watercare’s broader engagement framework.

Building for the future

Innovation and sustainability were embedded throughout delivery, supporting improved construction outcomes, long-term asset performance and social value.

The project achieved the highest Infrastructure Sustainability Council (ISC) Leading accreditation for the design phase. It was also awarded the 2026 Best in Class Arthur Mead Award (Infrastructure) at the Engineering New Zealand Te Ao Rangahau Auckland Branch Awards and named a finalist at the ISC Awards in the Excellence in Social Outcomes and Outstanding Achievement – IS Rating Scheme categories.

Key initiatives included HDPE liners and glass reinforced plastic to extend asset life, repurposing temporary works as permanent infrastructure and introducing electric trucks that reduced emissions by 79% compared to diesel equivalents.

Social value and community outcomes

Social value was embedded throughout delivery, with a focus on education, workforce development and partnerships with Māori and local communities.

The TupuToa Internship Programme provided targeted summer internships for Māori and Pasifika students aligned to project teams, offering practical experience in large-scale infrastructure delivery. The Dig Deep programme delivered numeracy and literacy training for the workforce, supporting long-term skills development.

In partnership with Make Give Live and May Road School, a wellbeing initiative supported student comfort during colder months through the provision of slippers and beanies.

A partnership with Mana Whenua hapū Te Ahiwaru established an on-site PPE laundry service in Māngere, improving hygiene, reducing risk for workers’ families and creating local employment and training opportunities.

Support for the Morningside Urban Market Garden Grow Space initiative enabled migrant and refugee women to develop gardening, business and financial skills, strengthening social connection and pathways to financial independence.