Photograph looking across Auckland city; Photograph looking across Auckland city;

Central Interceptor, Auckland

New Zealand’s largest wastewater project

Arup is taking on a key role for the largest wastewater project in New Zealand’s history, which is designed to safeguard the environment and people of Auckland.

Auckland, the largest city in New Zealand, is planning for the water needs of another one million people over the next 30 years. Being built by the local water authority Watercare, the new Central Interceptor Tunnel, a wastewater tunnel, will provide resilience to the current wastewater pipeline under the Manukau Harbour, as well as help Auckland manage the needs of a growing city and improve environmental outcomes at the same time.

Project Summary


14.7km long tunnel with 18 deep shafts

NZ$1.2billion

110 metresbelow the surface at its deepest point

Infographic showing Central Interceptor project key stats by Arup Infographic showing Central Interceptor project key stats by Arup
The new underground tunnel, which, at 14.7km long and 4.5m wide, is the largest wastewater tunnel in the country.

Arup is a world-leader in wastewater tunnel-design. We have selected them since the tender stage to support our Central Interceptor team to deliver innovative and cost-effective solutions. We were extremely pleased about Arup’s flexibility and responsiveness in accommodating our ideas and methodologies into the design. For this reason, we had no doubt in continuing to work with Arup for the detailed design stage. ” Francesco Saibene Director, Ghella Abergeldie Joint Venture

© Watercare

Ghella Abergeldie Joint Venture (GAJV) was appointed in early 2019 as Main Contractor by Watercare, with Arup commissioned by GAJV to provide tender design support, detailed design services and construction phase support of shafts structural design and the tunnel lining design.

The new underground tunnel, which, at 14.7km long and 4.5m wide, is the largest wastewater tunnel in the country. The 190m long tunnel boring machine, named Hiwa-i-te-Rangi (which means vigorous growth), begins her underground journey in early 2021. Due for completion in 2025, the Central Interceptor will connect Mangere Wastewater Treatment Plant to Grey Lynn by crossing the Manukau harbour at depths of about 15m below the seabed, with peaks of depths of 110m below the surface along its route.


Project challenges include the design of a one-pass tunnel lining system to ensure 100 years of design life, design of tunnel lining considering seismic loading condition, tunnelling under Manukau Harbour, as well as designing an effective inspection and quality control proposal to minimise human involvement in the wastewater environment.

Watercare team unveiling the tunnel boring machine in Auckland Watercare team unveiling the tunnel boring machine in Auckland
This tunnel boring machine will excavate the 14.7km long tunnel. © Watercare

Cascade drop shafts will be constructed, and, to improve overall construction safety, precast shelves and dividing walls have been proposed as a design and construct solution by the GAJV and Arup team. In addition, within the soils the team has proposed a sunken Caisson Method to combine temporary and permanent works within one shaft lining to improve productivity and reduce risk profile.

The geology within the main tunnel alignment and link sewers primarily comprise moderately weathered to unweathered, extremely weak to weak, interbedded sandstones, siltstones and mudstones of the East Coast Bays Formation (ECBF), and to a lesser extent ECBF residual soils, and recent alluvium and Kaawa Formation soils belonging to the Tauranga Group.

The project scope includes:

  • A 14.7km long tunnel with a 4.5m internal diameter, excavated with a tunnel boring machine

  • Two 3.2km long pipe-jacked sewers, at depths ranging from 12m to 70m

  • 18 drop shafts and access shafts, between 3m to 12m internal diameter at depths of 12m to 80m   

  • Numerous connecting sewers, chambers, control facilities and air treatment facilities, and

  • A wastewater pumping station including dual cell D-wall (12m and 26m internal diameter, up to 38m deep). 

Illustration of planned works for the Central Interceptor, Auckland Illustration of planned works for the Central Interceptor, Auckland
Illustration showing the placement of the tunnel and new facilities © Watercare

Arup’s design scope includes:

  • Design Management

  • General Geotechnical Design

  • Shaft Temporary Works and permanent lining design (Structural and Geotechnical)

  • Main Tunnel Lining Design

  • Link Sewer Jacking Pipe Selection and Design

  • Temporary Works Design for Tunnel Boring Machine operations for Main Tunnel

  • Digital Engineering

  • Construction Phase Support.

The scheme design and any other elements of the detailed design have been developed by Watercare’s consultant Jacobs.

Aerial image of excavation site for Central Interceptor, Auckland Aerial image of excavation site for Central Interceptor, Auckland
The excavation site © GAJV