New Zealand’s coastal waters face an escalating threat from exotic caulerpa, an invasive seaweed now confirmed at Waiheke Island’s Onetangi Bay and Thompson’s Point. This pest, first detected in 2021 at Aotea Great Barrier Island, has since spread across a growing 1,500 hectares in the Upper North Island including Ahuahu Great Mercury Island, Te Rāwhiti Inlet in Northland, Kawau Island, Waiheke Island, Mokohinau Islands, Rakino Island, Fantail Bay, Coromandel Peninsula, and Omaha Cove/Leigh Harbour. Caulerpa’s dense underwater mats disrupt native ecosystems, marine activities, and biodiversity. Biosecurity measures, including cleaning anchors and fishing gear, are crucial to halting its spread.
Collective action is essential to safeguarding New Zealand’s marine environment for future generations. Boaties play a crucial role in preventing its spread by cleaning anchors and gear. With caulerpa’s ability to devastate marine habitats, urgent biosecurity measures are essential to protect New Zealand’s waters for future generations.
When Northland realised the extent of its caulerpa infestation problem in 2023 it realised it faced a decade-long battle against invasive caulerpa, threatening marine biodiversity, fisheries, and tourism. (As do all places that caulerpa grows.) The seaweed spreads rapidly, smothering native habitats, depleting fish stocks, and disrupting traditional kaimoana areas. Although restrictions on anchoring and fishing are able to curb its spread, experts warn of severe long-term ecological and economic consequences without urgent intervention.
In late 2023, Northland authorities and local iwi planned with the New Zealand government to launch a large-scale dredging operation to combat the spread of invasive caulerpa in the Bay of Islands. Funded by Biosecurity New Zealand, the $1 million initiative included $800,000 for a specialised suction dredge, developed by Ōpua-based marine contractor Johnson Bros. Unlike traditional dredging, this method minimised harm to marine life. At the time authorities stressed the urgency of eradication, warning that caulerpa threatens biodiversity, kai moana access, and Northland’s marine way of life.
Late last month (February 2025) Northland Regional Council secured $6.2 million in government funding to advance its fight against invasive exotic caulerpa, particularly through a large-scale suction dredging operation at Omakiwi Cove, Te Rāwhiti. The funding announcement coincides with new discoveries of the seaweed at Urupukapuka, Motukiekie, and Moturua Islands, raising fresh concerns about its spread.
The suction dredging technique Bros trialled last year successfully cleared two hectares of caulerpa. With this funding, the council plans to develop an advanced submersible dredge planer (SDP), a remotely operated system designed to efficiently remove caulerpa in a single pass. Unlike the current barge-based dredge, the SDP will be faster, more precise, and less affected by poor weather. Operational testing is expected to begin in September.
As valuable as these efforts prove, caulerpa continues to spread, likely via boat anchors, chains, and fishing gear. Authorities urge boaties and fishers to check and clean equipment before moving locations to prevent accidental transport. Any suspected sightings, including washed-up seaweed, should be reported to Biosecurity New Zealand via 0800 809 966 or report.mpi.govt.nz.
Council Chair Geoff Crawford emphasised the importance of ongoing government investment and collaboration with local mana whenua partners, Ngāti Kuta and Patukeha hapū, who have been involved since caulerpa’s initial discovery in Northland. The next steps for managing the Urupukapuka outbreak are still being determined, with Biosecurity New Zealand assessing the necessary response.
With caulerpa now found across multiple popular boating areas, the council is reinforcing public awareness and the urgent need for preventive actions to safeguard Northland’s marine ecosystems and coastal economy.
Crawford warns that exotic caulerpa can easily hitch a ride on anchors, chains, and diving or fishing gear, leading to its unintentional spread to new locations.
To prevent this, he urges boaties and fishers to take simple precautions:
- Before departing, check your anchor, chain, and any equipment used in the water for tangled seaweed.
- Even if using an automatic retrieval system, inspect it for any attached fragments.
- If seaweed is found, remove it, bag it, or securely contain it to prevent it from re-entering the water, and dispose of it in a rubbish bin onshore.
- If secure containment isn’t possible, return it to the same waters it came from.
Upon returning to shore, any seaweed found on board should be disposed of properly in the rubbish.
Crawford also encourages reporting sightings of exotic caulerpa, including any washed up on beaches.
If possible, take a photo and record the location, then:
Call Biosecurity New Zealand at 0800 809 966 or
Submit a report online at report.mpi.govt.nz.
For further details on exotic caulerpa and current regulations, visit www.biosecurity.govt.nz/caulerpa.
