Current State of the Horowhenua (Waiopehu) Freshwater Management Unit

The Waiopehu Freshwater Management Unit (FMU) includes the Punahau Lake Horowhenua, Wairarawa (Waitārere), Waiwiri, Ōhau and Waikawa catchments in the south of the Horizons Region. Waterways in this FMU face pressure from intensive land use activities such as agriculture and urban development, wastewater and stormwater discharges, water abstraction for human use, and invasive freshwater weeds.
Lakes and estuaries are particularly sensitive to pollution. Punahau is a shallow coastal lake in the FMU that receives water from several streams, groundwater and drains that run through and under the Levin township and agricultural land.

In pre-European times, Punahau was a clean water supply, local taonga and valued fishery for the Muaūpoko iwi who lived in the coastal forest that surrounded the lake. However, the clearance of coastal forest, draining of swamps, intensification of land use, urban expansion, and the disposal of treated effluent in the lake between 1962 and 1987, led to significant degradation.

Punahau continues to receive large amounts of nutrients and sediment in addition to the ongoing internal processes influenced by past discharges into the lake. Water quality is poor as a result and the lake is regularly closed for recreation in the summer due to the potential presence of toxic cyanobacteria.

These are the issues Horizons is aiming to address with projects like the Arawhata constructed wetland complex.

The lower reaches and estuaries of the Ōhau and Waikawa streams are also vulnerable and suffer from eutrophication as a result of upstream land use activities.

Horowhenua (Waiopehu) Freshwater Management Unit Water Quality Model

The video below gives more information about the Waiopehu FMU and some of the science behind the Horowhenua (Waiopehu) Water Quality Interventions package.

Environmental data

Horizons measures a variety of water quality parameters across the FMU. This data is provided to the public via the Land, Air, Water Aotearoa (LAWA) website. Click on a link below to explore the data for that catchment.

Punahau Lake Horowhenua

Punahau is a shallow coastal dune lake and the largest natural lake in the Manawatū-Whanganui Region. In 2013, Horizons Regional Council installed a continuous water quality monitoring buoy in the lake, and since then water samples have also been collected by boat or helicopter, usually on a monthly basis.

Lake Horowhenua Catchment Streams

Lake Horowhenua has a single outflow to the sea, the Hōkio Stream, and receives water from several waterways, including the Arawhata, Pātiki and Mangaroa Streams.


The Ōhau River flows from the confluence of two short rivers, the North Ōhau River and the South Ōhau River. The Ōhau initially flows north, turning west to the southeast of Levin. It reaches the Tasman Sea 10 kilometres to the southwest of Levin.


The Waikawa Stream meets the sea at Waikawa Beach in Horowhenua. It is a popular spot for recreation. There is weekly summer season monitoring at some spots.