Major Win for Lake Wānaka as 40km of Shoreline Cleared of Invasive Weed
“This milestone marks a real triumph for the beautiful Lake Wānaka ecosystem and for everyone who treasures this iconic place,” said Minister Penk.
- Country:
- New Zealand
A long-fought battle to reclaim Lake Wānaka from the grip of an invasive aquatic weed has reached a significant milestone, with 40 kilometres of shoreline now successfully cleared of lagarosiphon, also known as oxygen weed. This marks a watershed moment in the preservation of one of New Zealand’s most iconic freshwater lakes, according to Land Information Minister Chris Penk.
“This milestone marks a real triumph for the beautiful Lake Wānaka ecosystem and for everyone who treasures this iconic place,” said Minister Penk.
The victory is the result of over 20 years of persistent control efforts led by Land Information New Zealand (LINZ), in collaboration with local communities, environmental groups, regional councils, and scientific partners.
What is Lagarosiphon and Why is It a Threat?
Lagarosiphon major is an invasive freshwater weed known for its rapid growth and dense underwater mats. Left unmanaged, it can:
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Smother native aquatic plant species
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Choke recreational waterways
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Obstruct hydroelectric intakes and irrigation systems
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Degrade aquatic habitats and water quality
Its spread is often human-assisted, with tiny fragments capable of hitching rides on boats, trailers, and fishing gear. A single fragment is enough to start a new infestation.
Shifting the Line: A First Since 2013
For the first time in over a decade, LINZ has been able to shift Lake Wānaka’s containment line—the strategic boundary separating weed-infested and weed-free waters.
“This shift proves we’ve made meaningful progress. We’ve won a key battle in a very long war,” said Minister Penk. “But the goal is to keep pushing that containment line further south, toward Roys Bay and the Wānaka township.”
Roys Bay, located near the heart of Wānaka, remains a vulnerable zone and a future target for eradication efforts.
Innovation in the Fight: The Role of Hessian Mats
A game-changer in this operation has been the use of hessian mats—biodegradable sheets laid on the lakebed that block sunlight and suppress weed growth while allowing native aquatic vegetation to recover.
These mats, combined with diver-assisted hand removal and targeted herbicide application, have allowed authorities to clear previously infested shoreline and prevent regrowth in treated areas.
The strategy is low-impact, enabling the restoration of natural biodiversity without the widespread damage caused by some conventional chemical treatments.
A Team Effort: Community and Science United
Minister Penk praised the multi-agency collaboration and community vigilance that made this environmental achievement possible.
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LINZ led national coordination and funding
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Otago Regional Council provided scientific monitoring
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Community groups and volunteers ensured education and public buy-in
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Lake users were instrumental in checking and cleaning their boats and gear to prevent further spread
“The clearing of lagarosiphon is a testament to the sustained commitment of all involved,” said Penk. “This is what success looks like when communities, scientists, and government pull together.”
The Battle Continues: Vigilance Still Required
While this victory is cause for celebration, the war is not over. Authorities warn that re-infestation remains a constant threat without continued public awareness and rigorous preventive practices.
Minister Penk emphasised the importance of ongoing diligence by lake users:
“Even a tiny weed fragment can undo years of work. That’s why it’s vital for boaties, anglers, and recreational users to continue checking, cleaning, and drying their gear.”
The Ministry continues to monitor the lake and fund active management in key hotspots. Long-term goals include full eradication in strategic areas and improved lake-wide biodiversity.
Looking Ahead: A Model for Other Lakes
The success at Lake Wānaka offers a blueprint for weed management in other high-value freshwater bodies across Aotearoa. With climate change and increased recreational lake usage posing ongoing risks, adaptive and science-led strategies will be crucial in preserving New Zealand’s freshwater heritage.
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