Why Lake Wānaka is Being Sprayed for Invasive Lake Weed This May
Updated as of 25 May: LINZ have said the Diquat spraying has been delayed another 2 weeks. With a further update coming Monday 8 June.
From Monday 18 May to Friday 19 June 2026, a targeted aquatic weed control programme using diquat herbicide is scheduled to take place at several locations around Lake Wānaka, including Eely Point, Roys Bay, Stevensons Island, and the Wānaka Marina.
This is the kind of topic that draws strong reactions. Some people hear “herbicide spraying” and feel immediate concern. Others wonder why more isn’t being done to address the problem already. Both responses reflect genuine care for the lake, and both deserve a considered response.
The programme, managed by Land Information New Zealand (LINZ) as part of its national freshwater biosecurity responsibilities, targets Lagarosiphon major, an invasive submerged weed species that has become one of the most persistent ecological threats to many of New Zealand’s lakes and waterways. Left unmanaged, lagarosiphon can form dense underwater forests that outcompete native aquatic plants, alter lake habitats, interfere with recreation and boating, and fundamentally change how freshwater ecosystems function.
In short: this is not simply “a bit of weed in the lake.” It is an invasive ecosystem engineer. And unfortunately, it is very good at its job.
The challenge of invasive species management
Managing invasive freshwater weeds at the scale now seen in Lake Wānaka involves trade-offs. Different approaches each have limitations, and no single option is without risk.
- Mechanical harvesting can sometimes spread weed fragments and contribute to further spread.
- Hand removal is extremely labour-intensive and generally only practical for very small infestations.
- Biological control for aquatic weeds are still limited in their effectiveness.
And doing nothing is not a neutral option either — it simply hands the lake over to the weed over time.
Within this context, Diquat is one of the tools currently used in New Zealand for controlling large underwater weed beds quickly enough to keep pace with the scale and growth rate of lagarosiphon.
Regulatory agencies allow its use under specific consent conditions, and it is typically applied in targeted ways where other methods are unlikely to achieve the desired level of control.
This does not remove the need for careful consideration. Rather, it highlights the importance of ensuring that any intervention is well-managed, proportionate, and based on the best available information.
What is diquat, and how does it work?
Diquat is a herbicide that has been used in New Zealand freshwater systems for decades and is specifically registered for submerged aquatic weed control.
In this operation, diquat will be applied in a gel form directly onto target weed beds from boats. Once in contact with the plant, it damages the weed tissue by interrupting photosynthesis, effectively dehydrating the plant.
The application is designed to be highly localised and applied at extremely low concentrations. Guidance from organisations such as Earth Sciences NZ (formally NIWA) indicates that effective concentrations are typically less than one part per million in water.
Diquat also binds rapidly to sediment particles in the water, where it becomes biologically inactive and gradually breaks down over time.
As with any intervention, risk depends on a combination of factors including dose, exposure, and how the substance behaves in the environment. Based on existing regulatory approvals and decades of operational experience in New Zealand, diquat use for submerged weed control is generally considered to present a low risk when applied in accordance with consent conditions and label requirements.
It is entirely reasonable for community members to have questions or concerns about herbicide use in a lake that is central to local life, identity, and recreation.
Those wanting to understand more about lagarosiphon, the management tools being used, and the long-term strategy for Lake Wānaka can find detailed information in the Lake Wānaka Lagarosiphon Management Plan 2025-2035, published by NIWA for LINZ. The plan covers the ecology of lagarosiphon, the range of control methods available (including diquat, hand weeding, hessian bottom lining, and suction dredging), and the management zones and objectives for the lake over the next decade.
What residents and lake users need to know
The spraying operation itself is expected to take only 1 to 2 days within the wider operational window and will proceed only under suitable weather and lake conditions.
As a precaution, the public are advised not to swim, fish, irrigate, or use water from the immediate treatment areas for 24 hours following applications.
Signage will be installed at public access points and boat ramps before and after treatment.
Contractors will also assess weed conditions in the week prior to spraying to ensure the operation proceeds only if conditions are appropriate.
Maps of the treatment sites will be released following final site inspections.
The bigger picture
Lake Wānaka’s clear waters can often obscure the ecological pressures developing beneath the surface.
One of the challenges of freshwater conservation is that ecosystems can appear visually pristine while ecological pressures quietly accumulate out of sight. Invasive species are particularly effective at exploiting that gap between perception and reality.
Managing these pressures rarely involves perfect choices. More often, it requires weighing imperfect options and making careful judgements about how to reduce long-term ecological risk while minimising short-term impacts.
WAI Wānaka is not responsible for this programme, and this article is not an endorsement of any particular product or method. Our role is to support the community in understanding how freshwater management decisions are made, what evidence sits behind them, and what trade-offs are involved.
Diquat spraying is not a silver bullet, and it is not evidence that environmental management has failed. It is one tool among several, chosen for this programme, and used cautiously as part of a broader effort to manage invasive weeds, while longer-term approaches continue to evolve as new methods and knowledge become available.
In many freshwater systems, there is an ongoing challenge in deciding when to act, as delayed intervention can narrow available options and make impacts more difficult to address. In situations like this, there is a risk that waiting until a problem is clearly visible can mean the window for effective management has already passed.
This programme is being delivered by LINZ as part of its national biosecurity responsibilities. For updates on timing and treatment areas, residents can visit the LINZ website or contact 0800 638 943.