Lake Horowhenua’s $11.2 million wetland clean-up has broken ground near Levin and Horizons Regional Council says the site is a perfect spot for a wetland because it used to be one

Construction of a new $11.2 million wetland system on the southern edge of Lake Horowhenua has finally broken ground near Levin, in what Horizons Regional Council is calling a major step in the restoration of one of the country’s most polluted lakes.

The Arawhata Constructed Wetland Complex started physical works in mid-March, and the first phase is on track for completion by the end of June 2026 to meet the deadline tied to its central government Jobs for Nature funding. Once finished, the wetland will sit between the Arawhata Stream catchment and Lake Horowhenua, also known as Punahau, intercepting sediment and nitrogen before they reach the lake.

Horizons Regional Council fresh water and projects manager Logan Brown told RNZ the design centred on two basic jobs. “The main reasons behind the wetland are to help improve Lake Horowhenua, the water quality and aquatic health,” he said, with sediment reduction handled by a chain of ponds and traps and nitrogen removal handled by the planted wetland itself.

Brown said the southern end of the lake was an ideal location for the new system. “It’s the perfect spot for a wetland. It used to be a wetland until it was drained to put into production. We’re putting it back into a wetland, although slightly different to what it was,” he said.

Project manager Aydin Maxfield said work on the ground had moved past the topsoil stage and into earthworks. “They’re pulling up peat and they’re lowering the contours. That will eventually receive the treated water that’s had most of its sediment removed,” he said.

The lake itself is small but it sits in one of the most heavily developed dairy and horticulture catchments in the lower North Island. Levin’s untreated sewage was once piped directly into the lake, a practice that ended decades ago but whose nutrient legacy is still working its way through the sediment. Surrounding wetlands were drained for production around 60 years ago, and Lake Horowhenua has carried a reputation as one of the worst-rated freshwater bodies in New Zealand ever since.

The funding split for the new wetland reflects how restoration of this scale gets paid for. Central government provided $11.2 million through the Jobs for Nature Covid recovery package, with Horizons ratepayers chipping in another $1.3 million. The rest of the cost involves land acquisition. Horizons bought a farm at the southern end of the lake and roughly 70 hectares of that land will eventually be turned into a multi-stage filter system across phases 1 and 2.

The gap between funding announcement and ground-breaking has been long. More than five years passed between the original 2021 allocation and physical work starting earlier this year, with consenting, planning and a High Court challenge to the resource consents all needing to be resolved first. The legal challenge was dropped in 2025, and both phases were ultimately consented through fast-track legislation.

Brown said the planted areas would not be set-and-forget. “Some of the wetland plants will need a haircut every now and then to keep them functional and doing that nitrogen removal,” he said. The sediment ponds will also need periodic clean-outs as silt builds up, and the nursery-grown plants now waiting to go into the ground will be planted weather permitting.

Phase 1.1, the part of the project being delivered against the June deadline, is essentially the upstream sediment ponds and the first stage of the planted wetland. Phases 1.2 and 2 are fully consented but will only proceed as further budget becomes available. If completed in full, the system will treat almost all of the surface water flowing into the lake from the Arawhata catchment.

The project sits alongside a separate, longer-running effort to remove the dense weed mats that have choked the lake for years. Hundreds of tonnes of weed have already been pulled out under that work, and lowering nitrogen and sediment loads through the new wetland is expected to slow the regrowth.

For Levin and the wider Horowhenua district, the work matters beyond the immediate science. Lake Horowhenua is a taonga for local hapū and a long-standing recreational lake for the community, and its degraded state has been a source of frustration for generations. A functioning wetland filter will not return the lake to its pre-1960s condition overnight, but it should start to bend the curve on the two pollutants that have driven its decline.

Maxfield said the next visible milestone for locals would be water flowing through the first sediment ponds and into the new wetland by winter, with planting picking up as conditions allowed.

Have you fished, swum or worked at Lake Horowhenua in better days, or are you watching the restoration from the Levin community? Share your story in the comments below.