A biosecurity leader says she “hears and shares” long-running frustrations over weed wash-ups at Rotorua’s lakefront, as new modelling is commissioned to improve control efforts.
Toitū Te Whenua Land Information New Zealand (LINZ) is investing in a computer model to better understand how lake weed moves around Lake Rotorua and where it is likely to wash ashore.
The model will simulate water movement and make predictions using wind speed, air temperature, solar radiation, humidity, and river inflows and outflows.
LINZ biosecurity leader Tracey Burton said the technology would allow “more effective decisions” and better “target control efforts” to weed beds contributing to wash-up events.
“We know how frustrating weed wash-ups have been for the Rotorua community, particularly following storm events, and we hear and share that frustration,” Burton said.
However, she cautioned the modelling was “no magic bullet”, noting weed control remained “notoriously difficult”.
Herbicide treatments could still fail when conditions were unsuitable, such as during algal blooms or when plants were too dirty for spraying to be effective.
Earth Sciences New Zealand will develop the model and test its accuracy against data collected from a monitoring buoy on the lake.
Researchers will focus on storm events linked to major wash-ups, allowing them to trace where the weed originated from.
The work follows a major clean-up on November 12, when stormy weather dislodged a large bed of invasive weed - roughly the size of Rotorua’s CBD - sending about 780 tonnes to the lake surface.
The decaying material produced strong odours along the lakefront and contributed to low oxygen levels that killed hundreds of fish. Clean-up efforts took three weeks and cost local councils a combined $133,443.
Lake weed is managed using funding from LINZ, which contributed $350,000 to Rotorua Te Arawa Lakes for the 2025/26 aquatic weed control season. About $1 million has been allocated to Otago for similar work.
Decisions on where the control work is carried out are made through the Bay of Plenty Aquatic Pest Co-ordination Group (APCG), which includes Te Arawa Lakes Trust as lake owners, LINZ, Toi Moana Bay of Plenty Regional Council, Rotorua Lakes Council, the Department of Conservation, and Eastern Region Fish and Game Council.
Te Arawa Lakes Trust oversees 14 lakes and performs weed control duties for Lake Rotorua.
Trust environment manager William Anaru said the computer model was a “positive move” helping better target spraying efforts, but said funding remained a major constraint.
“It is a big issue,” Anaru said, warning current investment was “nowhere near enough for what we need to do”.
He said the last funding increase was about six years ago and had been “slowly decreasing” since.
Controlling the weed-affected area near Rotorua’s lakefront alone could cost between $1m and $1.5m, he said.
Lake Rotoiti and Lake Tarawera also faced weed issues and would require ongoing yearly investment to stay on top of the problem.
Anaru said the impacts were clear for the community, from dead fish to strong odours and degraded water quality. The trust would continue pushing for improved outcomes to better protect the lakes.
In a statement to Rotorua Daily Post, Rotorua Mayor Tania Tapsell said the computer modelling “will give valuable data”.
“I’m glad we’ll soon have information from LINZ to identify where lake weed is, so we can try to stop future issues.”
Tapsell said Rotorua Lakes Council were working with Te Arawa Lakes Trust, seeking to raise $200,000 between local and regional councils and other external funders.
“We would then seek match funding from Government for a total spray programme of $400,000.
“This would be focused on preventing future impacts, rather than just reacting to them, while we work with our partners to look for a longer-term solution,” she said.
Tapsell said she had a “productive conversation” with previous LINZ Minister Chris Penk.
Funding “hasn’t been enough”, and Tapsell said this had been “clearly communicated” to the Government.
She wanted support to “hopefully eliminate” weed-washups for good.
“This lake weed is a pest species and shouldn’t be in our lakes in the first place.
“We’ve put so much effort into creating a world-class lakefront and on any other day it’s a stunning place, but this problem needs to be prevented urgently,” she said.
Annabel Reid is a multimedia journalist for the Bay of Plenty Times and the Rotorua Daily Post, based in Rotorua. Originally from Hawke’s Bay, she has a Bachelor of Communications from the University of Canterbury.