Client: Wellington Water Limited

Duration: Due for completion in mid-2026

Background

Wellington Water on behalf of Porirua City Council has commissioned a large wastewater retention tank located just beside SH59 and Porirua Station to help address the issue of untreated wastewater overflows into Porirua Harbour during rainfall events. Overflows happen when wet weather increases the amount of water flowing through the network beyond the amount the pipes can carry.

The wastewater retention tank will capture and hold stormwater and wastewater at times of peak flow, then gradually release it at levels the network can carry to the wastewater treatment plant. This will contribute to a healthier harbour and coastal waters.

The project scope includes:

  • A 70m x 20m x 7m concrete storage tank half buried in the ground;
  • 12m diameter pump station, excavated to 8m depth;
  • 2no. weir chamber structures;
  • 100m long retaining wall alongside SH59;
  • 1km of large diameter wastewater pipes
  • Associated M&E scope with 2 large pumps coming from Germany.

Delivery Solution

This project is a combined effort between our Water and Transport & Infrastructure business units. The Water Projects team have been able to bring their expertise in water infrastructure and mechanical and electrical engineering, while the Transport & Infrastructure team are able to provide their experience in project management and heavy civils construction.

Outcome

This combination of skills and expertise has already had excellent outcomes for both the project and the client.

Value engineering opportunities on the project have generated significant savings for the client. 

These included:

  • Changing from pre-cast walls to in-situ  walls for the storage tank
  • Reuse of earthworks waste material
  • Constructability changes to the retaining wall

The project obtained consent to store 7000m3 of the excavated material at a nearby location for reuse in the backfill around the partially buried storage tank. This initiative has helped to save cost through material reuse, increase productivity through shorter turn- around times, reduced carbon emissions and highway network congestion through reduced offsite trucking, and reduced strain on already stretched local landfill capacity through reuse of a construction by product.

The bulk earthworks involved significant heavy vehicle movements through an active and well-utilised park & ride facility. Utilising and maintaining good relationships with the park & ride operator meant these works were completed without a single complaint from the public.