Highly invasive caulerpa seaweed found at Whangārei Harbour

The highly invasive seaweed pest caulerpa has been found at Whangārei Harbour for the first time.

Northland regional councillor Jack Craw said the one-metre-wide patch was found by divers conducting regular surveillance at Smugglers Bay, near the harbour entrance.

Craw said the caulerpa patch was treated by covering it with a mat and applying chlorine, the standard method for small infestations.

Follow-up dives had found no further spread but extra surveillance would be carried out as a precaution.

The marine pest was found on 4 June and treated the following day.

Craw said the early detection showed surveillance work by the council and DOC was paying off.

It also proved the value of having an in-house dive team, he said.

"Our teams acted quickly to contain and treat the site using proven methods. Taking a cautious approach is critical with caulerpa, which can spread from even very small fragments," Craw said.

"We know caulerpa doesn't move long distances on its own. This is most likely human-assisted spread, which highlights how important it is that everyone using our coastal waters plays their part."

Coastal central councillor Amy Macdonald said the invasive seaweed could be spread by fragments carried on anchors, chains, or fishing and dive gear.

"Protecting our harbour is a shared responsibility, and simple steps from boaties and fishers can make a real difference," she said.

Those steps included checking anchors, chains and equipment before leaving a site; removing any seaweed and securing, for example in a bag, it so it could not return to the water; and putting it in a rubbish bin on land.

Exotic caulerpa is already well established around Aotea Great Barrier, where it was first found in 2021, and in the Bay of Islands, where it was identified by a hapū kaitiaki in 2023.

The fast-growing seaweed can rapidly cover large areas of seabed, smothering shellfish beds and altering ecosystems.

A large-scale caulerpa removal trial is underway in the Bay of Islands using a remotely controlled underwater vehicle fitted with a suction dredge head.

A major die-off occurred last summer, both in the Hauraki Gulf and in the Bay of Islands, but Biosecurity NZ pest management director John Walsh said the weed was showing signs of coming back.

The dramatic biomass reduction did, however, provide a welcome breathing space for those trying to combat its spread.

The cause of the die-off was not known but in the Bay of Islands it was to thought to be related to Cyclone Tam, which battered the area in April 2025.

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