New Underwater Dredge Strengthens NZ’s Fight Against Caulerpa

Hoggard described the successful testing as an important milestone, saying the SDP is now ready to become part of New Zealand's growing range of tools for tackling exotic caulerpa infestations.

A custom-built underwater dredge designed to remove invasive exotic caulerpa seaweed has completed field testing in Northland, giving New Zealand another powerful tool to protect its marine environment from one of the country's most challenging marine pests.

Biosecurity Minister Andrew Hoggard said the submersible dredge planer (SDP) performed as expected during recent trials at Omākiwi Cove in the Bay of Islands, demonstrating its ability to operate effectively in New Zealand's coastal waters. The remotely operated machine was developed for Biosecurity New Zealand by the Northland Regional Council, which partnered with local engineering firm Johnson Brothers Ltd to design and build the specialised equipment.

Hoggard described the successful testing as an important milestone, saying the SDP is now ready to become part of New Zealand's growing range of tools for tackling exotic caulerpa infestations. The project forms part of the Government's $10 million investment announced in 2024 to develop new technologies capable of controlling the invasive seaweed before it spreads further.

Innovative technology built for New Zealand conditions

Unlike conventional dredging equipment, the SDP was designed specifically for local marine environments because no suitable commercial system existed anywhere in the world. Two major components—the remotely operated vehicle (ROV) and its control room—were sourced from Scotland before being integrated into the New Zealand-built system. The machine features a unique screw-propulsion system that allows it to move efficiently across soft, sandy seabeds such as those found in the Bay of Islands.

Controlled remotely from the surface, the dredge uses cutting blades to slice through dense mats of caulerpa before suctioning the removed material to the surface for safe disposal. The trials, carried out with support from the Cawthron Institute and Ngāti Kuta and Patukeha hapū, confirmed the machine's manoeuvrability, operational performance and functionality. Environmental assessments are continuing to evaluate any impacts associated with its use.

New tool expands New Zealand's biosecurity response

The SDP is the third major treatment technology developed through the Government's investment programme. It joins a UV-C light treatment, which damages caulerpa using ultraviolet light, and the Rehabitat enclosure system, which seals chlorine over affected seabed areas to eliminate the weed. Ironically, testing took place after much of the caulerpa at Omākiwi Cove had naturally declined, leaving only scattered patches instead of the dense infestations that originally prompted the machine's development.

Despite the limited seaweed available during the trials, researchers were able to measure the machine's operating speed and the area it can cover in a single day. Officials say the SDP is fully prepared for deployment if dense infestations return and could also be adapted to remove other invasive aquatic plants in both marine and freshwater environments.

Hoggard said having multiple proven treatment options strengthens New Zealand's ability to respond quickly to future biosecurity threats while protecting the country's unique coastal ecosystems.