Guest Post by Dave Rhodes
Visitors to Rotorua are used to a certain smell. The geothermal sulphur haze is part of the city’s character. But recently another odour has taken over the lakefront — the stench of rotting aquatic weed and dead fish. While the media focus has been on the impact to lakeside cafés, spas and tourism, what is really unfolding is a visible sign of
This is not just a Rotorua problem. It is a warning for freshwater across New Zealand.
A 300‑hectare weed bed and suffocating fish
Authorities have confirmed that an enormous submerged weed bed — around 300 hectares in size — stretches along part of Lake Rotorua’s shoreline. During strong northerly weather, vast sections tear free, drift inshore, and accumulate in thick mats along the foreshore.
As these mats rot, they strip oxygen from the surrounding water, creating hypoxic conditions that literally suffocate fish. Dead fish are being found tangled within decomposing weed. This is a classic symptom of an ecosystem under extreme stress.
For anglers, this means more than a foul‑smelling inconvenience. It signals habitat collapse, loss of invertebrate life, changes to spawning conditions and a breakdown of the ecological balance required to support a healthy fishery.
A lake once managed and celebrated for its trout and cultural significance is now experiencing regular, visible die‑off events.
A
Mechanical weed harvesters are operating daily, removing as much as 60–70 tonnes of weed at a time, while dozens of truckloads are carted away for disposal or processing. Similar efforts took place in earlier years — including the removal of hundreds of tonnes in 2022 and 2024.
Yet the underwater weed bed continues to expand.
This highlights a serious problem with New Zealand’s approach to freshwater management: systematic degradation is being met with temporary clean‑ups rather than structural solutions.
Instead of a long‑term, ambitious plan to remove or suppress invasive aquatic plants at scale — and tackle the nutrient conditions that enable them to thrive — authorities are locked into a costly loop of reaction, clean‑up and explanation.
It is essentially environmental triage.
Too many agencies, too little urgency
Lake Rotorua’s weed management is split between multiple organisations — LINZ, Bay of Plenty Regional Council, Rotorua Lakes Council, Te Arawa Lakes Trust, and DOC. Each has a role; none bears full responsibility for outcomes.
Funding is piecemeal. Decision‑making is slow. Priorities shift between amenity values, cultural values, tourism interests and environmental protection.
What is missing is a single, fully funded, long‑term intervention that treats invasive weed as an ecological threat, not a cosmetic problem.
To put it plainly: no one is being held accountable for improvement — only for response.
The didymo parallel: managing decline instead of stopping it
This situation echoes the arrival of didymo (rock snot) in the South Island in 2004.
Initially there was alarm and decisive language. But fairly quickly, authorities concluded that eradication in large river systems was “not possible”. The strategy shifted to containment — ‘Check, Clean, Dry’ — and a reluctant acceptance that many rivers would be changed forever.
In effect, we moved from eradication to managing the inevitable.
Today, didymo is widespread across major South Island rivers. A line has been drawn between ‘infected’ and ‘uninfected’ catchments, but the reality is that we normalised ecological downgrade rather than seriously confronting it in its early phase.
Rotorua’s weed problem follows the same pattern:
- Acceptance that total control is difficult
- Limited funding
- Reactive management
- Normalisation of ecological loss
This isn’t a failure of science — it’s a failure of political will, investment and imagination.
Could we just ‘sacrifice’ one river or lake?
From time to time a radical thought appears: Why not poison or sterilise one waterway completely to save the rest?
While this idea has emotional logic, in real terms it is neither practical nor ethical.
To truly eliminate an organism like didymo or invasive aquatic weed, treatment would have to penetrate every surface, crevice, sediment layer and tributary — without harming downstream ecosystems or people. A dose strong enough to kill the target species at scale would also destroy native plants, invertebrates, fish and entire food webs.
Even then, all it would take is a single reintroduction from a contaminated boot, trailer, bird or upstream spring for the problem to begin again.
The ecological, cultural and legal consequences of deliberately poisoning a river or lake in New Zealand would be immense — and rightly so. Our water bodies are taonga, not expendable test sites.
So while we shouldn’t — and legally couldn’t — “nuke” a river to fix it, the uncomfortable truth remains: we also aren’t doing nearly enough to prevent collapse through more balanced, long‑term interventions.
Tourism first, ecology second
Much of the public conversation centres on embarrassed retailers, uncomfortable visitors and poor online reviews. These are real and valid concerns — but they are symptoms of ecological breakdown, not the problem itself.
What is missing from the mainstream discussion is:
- What is happening to fish populations?
- How are spawning sites being affected?
- What does this mean for long‑term water quality?
- How widespread is oxygen depletion in the lake during these events?
Rotorua’s lakefront does not just need deodorising. It needs restoration.
What is needed:
This moment should not be allowed to pass as just another unpleasant summer. A responsible, foresighted government would
1. A fully funded,
A multi‑year plan (20+ years) for Rotorua and its sister lakes, focused on actual reduction of invasive weed biomass — not just harvesting what floats ashore.
2. Stronger national freshwater biosecurity
More resources for inspection, wash‑down infrastructure, enforcement and public compliance. Prevention is still the cheapest and most effective tool.
3.
Weed growth is fed by nutrients. Until that input is properly dealt with, harvesting is nothing more than a band‑aid.
4. Transparent monitoring and reporting
Regular public release of dissolved oxygen levels, plant biomass data, fish mortality events and ecosystem trend analysis.
5. Formal angler involvement
Anglers are constant observers of ecosystem change. Their data and experience should be formally integrated into management planning.
Conclusion: The smell will pass. The damage may not.
The wind will change. The piles will be removed. Tourists will return.
But unless underlying failures are addressed, the weed will grow back. The oxygen will crash again. The fish will die again.
Rotorua is giving us a very clear warning. The question is whether we will treat it like we did didymo — as an inconvenience to manage — or whether we will finally treat freshwater ecosystems as something worth genuinely protecting.
New Zealand cannot keep normalising degraded lakes and rivers and still pretend to be a clean, green nation.
For anglers, for wildlife, and for the next generation, that narrative must change — starting now.
You mention didymo and its spread. It was blundering by government and a lack of watchdog by NZ Fish and Game that allowed a jet boat marathon around South Island go ahead at the time of the didymo discovery in the Waiau as I recall.
must be a bad smell if residents are complaining!
I hope the stench is strongest near the home of Todd McLay, MP for the Rotorua electorate.
We might even see some action then (just like in 1858, when the Thames river was so foul that the smell interfered with proceedings in the English Parliament).