The team at Omākiwi Cove getting the $5m submersible dredge planer ready to go into the water. Photo / Susan Botting
The 16-tonne Northland-made ROV is a tethered robotic submersible controlled by surface operators, enabling large-scale caulerpa removal underwater without divers.
It was designed and developed by Johnson Bros under an NRC project funded by Biosecurity New Zealand, a business unit of the Ministry for Primary Industries.
“It’s a technically ambitious, innovative solution to a really big problem facing New Zealand that’s already significantly changing the Kiwi way of life,” Johnson said.
“If this had been built anywhere else in the world, it would have cost hundreds of millions of dollars.”
The red final prototype performed well during its first four hours of full operation on Tuesday, Johnson said.
It is designed to remove almost 1.5 rugby fields of caulerpa a day, a major leap from the roughly 70sq m initially removed using divers manually guiding a suction dredge.
“And we’re aiming for a really high, 100% complete removal in a single treatment,” Johnson said.
The machine is an underwater robotic mower and vacuum cleaner combined.
It crawls across the seafloor at about 8m a minute, shaving off the caulerpa plant, stolon and rhizomes, along with about 7cm of surface sediment, using a planing head that works on the same principle as a wood planer.
Caulerpa at Omākiwi Cove in the Bay of Islands. Photo / Northland Regional Council
The project is part of New Zealand’s fight against caulerpa, considered the world’s largest invasive seaweed removal programme.
An earlier version of mechanical caulerpa removal used equipment operating from a surface barge, but the final submersible carries much of its machinery underwater as it works.
Operators control the new ROV remotely from a control room on the barge that serves as the project’s nerve centre, displaying camera feeds, sonar data, GPS positioning and system performance.
Power, control signals and data are transmitted through a 650m umbilical cable to the ROV.
The ROV incorporates components from New Zealand, Scotland, Australia and Germany.
Repurposed equipment from a second-hand Scottish North Sea oil and gas industry ROV, originally designed to operate at depths of up to 350m, now forms the Omākiwi Cove machine’s onboard electric hydraulic power unit. This powers pumps, hydraulics and other moving parts through 250m of hydraulic cabling.
Connection points for myriad hydraulic cables help make the submersible dredge planer work. Photo / Susan Botting
The submersible was trucked to Omākiwi Cove on June 18 after being built in Ōpua over 18 months.
With a footprint of 52sq m, it is 13m long and 2.5m high, about the size of two tiny homes.
The early stages of New Zealand’s large-scale caulerpa eradication effort at Omākiwi relied on crews using a small hand-held suction nozzle to manually vacuum the pest seaweed.
Early mechanical dredging trials were promising, removing about 2500sq m of caulerpa a day. The final prototype can remove about 10,000sq m in the same period.
NRC biosecurity manager marine Dr Kaeden Leonard said the new ROV’s ability to remove large quantities of caulerpa in areas where it had spread into dense meadows was a potential game changer.
“This takes us from manual harvesting to large-scale mechanical removal,” Leonard said.
The ROV vacuums caulerpa from the seabed and sends it to the barge, where sand and water are separated and returned to the sea before the seaweed is taken ashore.
It moves across the seabed on two 7m rotating steel cylinders with corkscrew-shaped blades. Known as Archimedes screws, they propel the vehicle while spreading its weight to reduce the risk of bogging in soft sediment.
The two giant Archimedes screws under the submersible dredge planer that it uses to move along the sea floor. Photo / Susan Botting
Ballast can be adjusted within the screws to change how much weight they place on the seafloor, balancing traction with minimal disturbance.
Large inflatable yellow ballast bags are also attached to help float the ROV off the seafloor.
NRC marine biosecurity specialist and project lead Derek Richards said the new dredge planer could transform New Zealand’s response to major caulerpa infestations.
Andrew Johnson with the inflatable yellow bags that help with ballast for the new submersible dredge planer. Photo / Susan Botting
Exotic caulerpa has previously covered the equivalent of 115 rugby fields in the Bay of Islands and more than 1100 rugby fields around Great Barrier Island.
Despite significant dieback in both locations, Richards said, the machine remained a crucial tool in New Zealand’s battle against the invasive seaweed.
“We never really know what’s going to happen next with new invasive pests,” he said.
NRC marine biosecurity specialist Derek Richards, the council’s project manager for the new dredge planer, with the buoyancy control panels that sit on the barge above the planer. Photo / Susan Botting
Caulerpa grew in dense meadows at Omākiwi Cove, forming thickets up to half a metre high that covered the seafloor. These began dying back in 2025 after sediment from Cyclone Tam covered large areas.
Richards said other dieback causes were still unknown, but the seaweed was regrowing.
Environmental monitoring was carried out before launch, and results will continue to be assessed as trials progress, Richards said.
■ LDR is local body journalism co-funded by RNZ and NZ On Air.