22 July 2026
Kia ora tatou. In this issue:
Updates on operational work at Bay of Islands, Hauturu and Coromandel
Submersible dredge planer trialled in-water
Connecting with international sailors
- Coming up – Revive Our Gulf Community Forum.
Submersible dredge planer in-water trials
The third and largest of three tools funded through a $10 million government allocation has undergone in-water trials at Omākiwi Cove in Northland.
Northland Regional Council received a Biosecurity New Zealand grant to develop the submersible dredge planer to remove extensive mats of exotic caulerpa from the seafloor. The two other tools developed through the programme are the Rehabitat chlorine treatment enclosure and a UV-C light array. These three tools form part of a growing toolkit, each designed for use in different conditions and environments.
Northland engineering company Johnson Brothers Ltd designed and built the $6.3 million submersible dredge planer under contract to Northland Regional Council. Scientific oversight is being provided by the Cawthron Institute, and local iwi Ngāti Kuta and Patukeha hapū supported the project. Key components, including the remotely operated vehicle and control room, were sourced from a specialist company in Aberdeen, Scotland.
Pictured above: Project partners and stakeholders alongside the assembled submersible dredge planer during a site visit in late June.
The device moves across the seafloor using large screw-like propellers. As it travels, it cuts through dense carpets of exotic caulerpa, which are then suctioned to a support barge on the surface for collection and disposal.
Recent in-water testing showed it met performance expectations. This did not include actual harvest of exotic caulerpa because of biomass dieback in the area.
The submersible dredge planer was conceived at a time when large caulerpa meadows were expanding rapidly, and there were concerns infestations could continue to grow exponentially. More recent observations suggest exotic caulerpa may follow a boom-and-bust cycle in some locations, but the ability to respond quickly to extensive infestations remains important. The machine also has the potential to support responses to other aquatic biosecurity incursions.
Removal trials on rocky reefs at Hauturu
Important trial removal work has been done this winter around Hauturu-o-Toi Little Barrier Island to deal with a patchy infestation on rocky reefs around the island.
A partnership group has shaped, funded and organised delivery of this work, made up of: Ngāti Manuhiri, Biosecurity New Zealand and Auckland Council.
The trials are investigating how treatments perform on rocky reef, in contrast to the sandy substrates elsewhere.
Two methods have been tested on parts of the Hauturu infestation: Advanced Aquarium Technologies’ UV-C light array in the form of a smaller “rock and reef unit” and a smaller, 1-square-metre version of Commercial Dive Services’ Re-habitat treatment chamber.
The Cawthron Institute has provided scientific oversight and design for the work, with a formal report on the trial to be received in the coming weeks.
Removal trials at Tāwharanui Marine Reserve
There’s also been good progress at Tāwharanui Marine Reserve, to the west of Hauturu.
Upon notification of exotic caulerpa being detected there, the Department of Conservation (DOC), Biosecurity New Zealand and Ngāti Manuhiri together formulated a bespoke approach to tackle the first discovery of exotic caulerpa in a marine reserve.
DOC undertook extensive surveillance and identified areas suitable for hand-held suction dredge removal, which was funded through Biosecurity New Zealand.
Cawthron Institute provided scientific oversight, monitoring the success of the removal and any impact on the surrounding environment. DOC will undertake monitoring in the coming months, including assessing any ongoing impacts from exotic caulerpa.
Extraction system being fine-tuned in Northland
A Northland dive team is achieving encouraging results with a targeted caulerpa extraction system.
In November 2025, Foundation North answered the call from the Conquer Caulerpa Charitable Trust and provided funding for treatment of patches of exotic caulerpa that had been located outside the Controlled Areas enacted by Biosecurity New Zealand. The new funding enabled 96 days of dive work by the team from Northern Divers NZ Limited, and Biosecurity New Zealand provided additional funding and support for the work.
“Our aim was to prevent the smaller patches from developing into meadows of caulerpa,” says Richard Anyon, Trust Chair. “The divers have been going into the water, doing the mahi. They started at the outer limits and have been working back. They are making significant progress. In the past six months the Trust’s dive team has carried out 634 transects and swam nearly 300km. This work has stopped caulerpa from growing into meadows.”
After initially using benthic mats with added chlorine, the dive team then piloted an extraction system that avoids the use of chlorine and targets caulerpa without disrupting surrounding ecosystems. The deployment of the extraction system was funded by Biosecurity New Zealand.
A diver moves an extraction head around to remove caulerpa from the seabed and the calibrated system pumps it up to bags on a boat above. Northern Divers have been fine-tuning the settings for the extraction and pumping, as well as the fineness of the bag mesh to ensure it holds any fragments. Extracted exotic caulerpa is taken to a landfill.
Pictured above: The Northern Divers dive team.
The amount cleared per day depends on how dense or patchy the exotic caulerpa is. At one location at Motukiekie, the divers used a swim-line as a reference, working five metres each side, and cleared 400 square metres of seabed in a day. They removed patches ranging from single fronds to dense patches of up to half a square metre. In March, at Long Beach, where infestations were generally sparse and of medium density, they cleared 1,900 square metres in eight patches in a single day. At Wairaka Bay they cleared 1,000 square metres in a day, removing eight large patches, with the largest measuring 1.5 square metres and of medium density.
Since initial work at Paroa Bay and Motukiekie Channel in the first week of January, the dive team has revisited the treatment sites every 30 days. So far, several months post treatment, there is no regrowth.
“The dive team are currently actively removing large infestations in Otaio Bay using the extraction tool - this tool allows removal while leaving taonga species intact,” Richard reports. “It has become evident how quickly small infestations left unchecked can quickly spread. Ongoing funding sources are imperative if we are to manage the spread within the Bay of Islands.”
Coromandel east coast update
This season’s monitoring programme across the east coast of Coromandel with Ngāti Hei and their dive partner Coast & Catchment has provided an important update on the spread of exotic caulerpa in the area.
Dive surveys carried out between December 2025 and March 2026 at Repanga - Cuvier Island, Korapuki, Port Charles and nearby islands found that exotic caulerpa remains present but patchy. In these areas it is living alongside native species such as kelp, sponges and coralline turf rather than forming dense, single-species beds.
The abundance of exotic caulerpa increased over the summer months: while no exotic caulerpa was detected at Repanga in December, follow-up surveys in March identified 23 patches along the island’s western coastline and a further 39 patches were recorded at Korapuki, indicating that exotic caulerpa is becoming more established in some areas.
Encouragingly, no exotic caulerpa was found at Green Island, highlighting that some parts of the rohe remain unaffected.
Pictured above: Exotic caulerpa observed at Repanga (Source: Coast & Catchment Ltd).
Overall, the findings suggest that while exotic caulerpa is spreading in parts of the eastern Coromandel, it is not dominating reef ecosystems or causing obvious ecological harm.
The programme partners also trialled a control method using small biodegradable woollen mats placed over affected seabed areas. At Port Charles, this approach successfully eliminated all exotic caulerpa beneath the mat within approximately eight weeks, marking a significant step forward in local management tools. However, the surveys confirmed that across large and complex reef systems such as at Repanga and Korapuki, completely removing exotic caulerpa is not currently practical due to the type of environment and the difficulty of locating every small patch.
The focus in future will be on ongoing monitoring, protecting exotic caulerpa-free areas and targeting key hotspots, especially during late summer when exotic caulerpa is easiest to detect. This work will help ensure that Ngāti Hei and partners can respond early to changes and continue to safeguard the health of local marine environments.
Reaching international sailors
Exotic caulerpa was included in Biosecurity New Zealand's pre-departure briefing for participants in the annual Pacific Rally organised by Island Cruising NZ in April.
The briefing highlighted the potential impacts of exotic caulerpa, outlined Controlled Area requirements and encouraged sailors to follow the ‘Bag It, Bin It’ call to action to help prevent its spread.
Biosecurity New Zealand provides marine biosecurity information to international sailors in a number of other ways, including a double-page spread in the annual Gateway magazine and a Protect Our Paradise pack that is distributed to arriving vessels following biosecurity inspections by border staff.
Pictured above: Biosecurity New Zealand representative Gabrielle North tells Pacific Rally sailors about exotic caulerpa.
COMING UP – Revive Our Gulf Community Forum, Tues 28 July
The next quarterly Revive Our Gulf Community Forum on Exotic Caulerpa will be held online on Tuesday 28 July. Presentations will include research updates from Biosecurity New Zealand. Use this link to sign up for this hui.
Nāku noa, nā – The Exotic Caulerpa Programme team on behalf of the exotic caulerpa management partners.
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For more information about exotic caulerpa seaweeds in New Zealand visit the Ministry for Primary Industries Biosecurity New Zealand website.