Auckland skyline at dawn; Auckland skyline at dawn;

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


13kilometres long tunnel with 10 deep shafts

4.5metresdiameter tunnel

110metresbelow the surface at its deepest point

Infographic on Central Interceptor, Auckland, New Zealand Infographic on Central Interceptor, Auckland, New Zealand
The new underground tunnel, which, at 13km long and 4.5m wide, is the largest wastewater tunnel in the country. © Watercare

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 13km long and 4.5m wide, is the largest wastewater tunnel in the country. Due for completion in 2025, the Central Interceptor will connect Western Springs and the Mangere Wastewater Treatment Plant 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 Tunnel Boring Machine Watercare Tunnel Boring Machine
The 13km long tunnel will be excavated by a tunnel boring machine. © 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 13km 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

  • 16 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). 

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.