Project overview

The Maitai (Mahitahi) River is a treasured taonga for Wakatu Nelson, valued for its cultural, recreational, and ecological significance. Historically, water discharged from the Maitai Reservoir, Nelson’s primary water supply, exhibited poor quality, threatening river health and undermining its cultural and recreational value.

To address this, Nelson City Council initiated the Maitai Reservoir Aeration project to meet new resource consent requirements which aimed to improve ecological health of the river. The objective was to increase dissolved oxygen (DO) levels in the reservoir, which depletes due to thermal stratification during summer months. By enhancing natural aeration processes, the system improves water quality and supports a healthier river ecosystem. Tonkin + Taylor led a team which included specialist subconsultants to deliver the services outlined here.

Maitai Dam

Services

  • Hydrodynamic water quality modelling – development and validation of reservoir model
  • Environmental data analysis and consent support – including modelling for consent variation
  • Civil engineering design – sparge line layout and reservoir integration
  • Mechanical engineering design – compressor and aeration system specification
  • Electrical engineering design – power supply and control systems
  • Structural engineering design – support structures for aeration equipment
  • Procurement and tender management – contractor engagement and selection
  • Construction supervision and quality assurance – oversight during installation
  • Health and safety in design reviews – risk assessment and mitigation planning
  • Project management – coordination of multidisciplinary teams and timelines

Client

  • Nelson City Council

Year

  • 2019 – 2025

Maitai Aeration System

Maitai Dam

Our Role

Tonkin + Taylor led the modelling, planning, design, procurement, and construction supervision for the project with specialist subconsultant support.
Key services included:

  • Hydrodynamic water quality modelling
  • Civil, mechanical, electrical, and structural engineering design
  • Procurement and construction supervision
  • Stakeholder and iwi engagement support

Technical Solution

The project began with the development and validation of a hydrodynamic water quality model to simulate reservoir conditions and assess aeration options. After comparing hypolimnetic and bubble plume aeration, bubble plume aeration was selected. Modelling informed system specifications, which guided detailed engineering design.

 System Specifications:

  • Two duty/standby 75kW oil-free air compressors
  • Variable air flow up to 240 L/s
  • 430m submerged sparge line suspended above the reservoir bed

Commissioned in December 2024, the system has already exceeded performance expectations, delivering measurable improvements in DO levels and overall river health.

Collaboration + Community Engagement

In Te Reo Māori, Mahitahi means “working as one”, a principle that guided this project. Nelson City Council led stakeholder engagement, consulting with Te Tau Ihu iwi to ensure cultural oversight and uphold kaitiakitanga values. The iwi visited the site during construction, ensuring the project protected the mauri of the Maitai Awa.

Other key collaborators included:

  • Friends of the Maitai – local environmental group aligning the project with community aspirations
  • Cawthron Institute – scientific validation and ongoing monitoring
  • Fulton Hogan – contractor providing early input and leading construction with local subcontractors

Engineering New Zealand hosted a post-completion site visit to share knowledge and promote innovation across the sector.

Environmental + Social Impact

The aeration system directly addresses long-standing water quality issues caused by thermal stratification, restoring DO levels and improving ecological health. Key sustainability features include:

  • Modelling-led design for efficiency of aeration
  • Energy-efficient compressors with low operational footprint
  • Reduced iron and manganese concentrations in discharge water
  • Improved aquatic ecosystems and reduced nuisance algae growth

Beyond technical success, the project delivers cultural, recreational, and social benefits:

  • Cultural: Enhances river mauri and supports iwi values of kaitiakitanga
  • Recreational: Cleaner water for swimming and outdoor activities
  • Social: Strengthens community stewardship and trust
  • Educational: Knowledge sharing through industry engagement
  • Operational: Lower treatment costs and reduced carbon footprint

Key Outcomes

  • Significant improvement in DO levels since commissioning
  • Healthier aquatic ecosystems and reduced contaminant concentrations
  • Replicable model for sustainable water infrastructure
  • Strong alignment with cultural and community values

Maitai Team Training

Maitai Sparge Line

> Dam Engineering

> Dam + Hydro Capability Sheet

> Learn about Damtec – a digital platform to manage dam safety

Explore More Projects

LEAD Housing Development Programme, Auckland

LEAD Housing Development Programme, Auckland

Drury South Crossing Precinct

Drury South Crossing Precinct

previous arrow
next arrow

How can we help?

Get in touch with a Tonkin + Taylor representative today. Our friendly and knowledgeable team make it easy to find the right solution to suit your specific needs.

Get In Touch