Waihī Estuary constructed treatment wetlands

Waihī Estuary’s water quality has declined in recent decades due to drainage of wetlands, channelisation of rivers and streams, and land use intensification.

To reduce the decline and improve water quality entering the estuary, Toi Moana Bay of Plenty Regional Council has partnered with local landowners and Earth Sciences New Zealand (formally NIWA) to build and monitor two treatment wetlands: Te Rere I Maniatutu and the Pongakawa wetland.

These wetlands are two of six constructed around the country that are part of Earth Sciences New Zealand’s national programme to monitor the effectiveness of wetlands to reduce contaminants as part of the research project.

Why did we do this?

The Waihī Estuary has been identified as one of the most degraded in the country. Monitoring and modelling work shows it needs substantial reductions of 40-60% in sediment, nutrients and pathogens, plus in-estuary interventions, to achieve a state of moderate health.

Key benefits of these wetlands include:

Capturing sediment from the water to stop it from washing out into the Waihī Estuary
Improving water quality by removing nutrients and E. coli from the water
Providing a place for fish, insects, invertebrates, and birds to flourish
Retaining water during heavy rainfall events.

Te Rere I Maniatutu Wetland

Previously a drain for a dairy farm, Te Rere I Maniatutu wetland is a partnership between the Regional Council, kiwifruit company Baygold, Earth Sciences New Zealand and Ngāti Pikiao, to slow the flow of water off the land and reduce contaminants entering the Waihī Estuary.

About the project

Initially constructed in 2022, before expanding in 2024, Te Rere I Maniatutu Wetland stretches over approximately 3ha on the corner of State Highway 2 and Maniatutu Road.

This wetland was designed as a series of inter-connected ponds. Each pond has smaller sections of deeper water to allow sediment to settle, larger areas of shallow water for densely vegetated plants, and open areas to allow the water to recirculate and redistribute across the wetland.

It has been planted with more than 41,000 natives, with margins and riparian surrounds a mixture of sedges, flaxes, shrubs and trees. As well as improving water quality, the goal is to boost biodiversity and enhance the cultural values.

This project was made possible through funding from the landowner (Baygold) and Regional Council, with support also provided by the Ministry for the Environment and the Ministry for Primary Industries.

Pongakawa Constructed Wetland

The Pongakawa Constructed wetland is a partnership between the Regional Council, Pax Hickson Trust, Marmatsar Trust and Earth Sciences New Zealand. The wetland was designed to remove contaminants and treat water from the surrounding 48-hectare catchment of dairy and orchard land.

About the project

The one-hectare Pongakawa wetland (which was completed in 2022) is located on a low-lying, flood prone retired pasture near the Puanene Stream. The Hickson family (Pax Hickson Trust and Marmatsar Trust) provided the land on their dairy farm and co-funded the project.

The wetland includes sections of deep water to allow sediment to settle, larger areas of shallow water with plants, and open areas to slow the water, helping to maximise nutrient removal through the wetland before eventually reaching the estuary.

More than 20,000 native sedges, flaxes, shrubs, and trees were planted.

This project was made possible thanks to funding from the Regional Council, the Ministry for Primary Industries and the Ministry for the Environment.

What's happening now?

With both wetlands constructed, we are focused on monitoring water quality and contaminant loads entering and exiting the wetland.

Monitoring results for Te Rere I Maniatutu wetland from 2024 show that the original two-hectare wetland has already reduced contaminant concentrations significantly, outperforming predicted figures.

Nitrate nitrogen: 89% average reduction
Ammonium nitrogen: 85% average reduction
Dissolved reactive phosphorus: 90% average reduction
E. coli: 80% average reduction
Total phosphorus: 75% average reduction
Total nitrogen: 55% average reduction
Total suspended solids: 43% average reduction

Building on the successful performance of these wetlands, a much larger treatment wetland project is now underway alongside Waihi Estuary (Transformational joint venture to restore Waihi Estuary).

Other resources

Te Rere I Maniatutu and Pongakawa wetland factsheet

News and updates

A MONTH AGO Constructed wetlands project receives national award for environmental sustainability

An innovative project constructing new wetlands to improve water quality has been recognised at the recent International Water Association (IWA) and ASPIRE Water New Zealand Conference in Christchurch.

Read the full media release.

3 MONTHS AGO Constructed wetland guidelines

Constructed wetlands are a water quality restoration tool that can reduce levels of sediment, nutrients and microbes such as E. coli.

This guide provides design and performance information for people wanting to establish a surface-flow constructed wetland to specifically reduce contaminant loss (nitrogen, phosphorus and sediment) from subsurface tile drains, shallow groundwater from seeps and springs, and surface drains and small streams in pastoral farming landscapes.