It was only a matter of time. In May, kaitiaki Rana Rewha (Ngāti Kuta) found 10 small clumps of fluffy green algae on the beach at Omakiwi Cove in the Bay of Islands, and raised the alarm.

The clusters were exotic caulerpa—the invasive tropical seaweed first discovered at Aotea/Great Barrier in 2021, now detected on the mainland. Divers searched the waters nearby and soon found swathes of it at a dozen sites, the 16-hectare infestation suggesting the underwater weed had been quietly trespassing for several years already. Later, extensive surveillance dives discovered 200 hectares in the Bay of Islands had been infiltrated to varying degrees.

This month, Biosecurity New Zealand and mana whenua declared a Controlled Area Notice and rāhui over Te Rāwhiti, banning fishing and anchoring in an effort to stop the spread.

“But no one’s rocking up there today, with big salt bags, big tarpaulins, big suction dredges—there is no mechanism in place to get rid of it,” says Craig Thorburn from the Waiheke Marine Project.

Previously, caulerpa response director John Walsh has said that eradication is untenable with current tools, and containment the only realistic option.

But unrest is growing over the government response to the crisis. Seven North Island councils are calling for more money and support for both surveillance and eradication efforts. Mussel-restoration group Revive Our Gulf told ministers caulerpa should be treated like an oil spill or mycoplasma bovis—and funded accordingly.

Lessons from overseas offer both foreboding and hope, Thorburn says. In Monaco, one tiny caulerpa patch spread to six Mediterranean countries and after 16 years carpeted 131 square kilometres—nearly half the size of Aotea. After just six years, fish stock dropped by half.

But authorities in Newport, California, have successfully eradicated small outbreaks three times. When the latest was detected, a team was in the water eradicating within 17 days. We need a national rapid-response strategy here too, Thorburn says, ready to swing into action wherever caulerpa arrives next—imagine what it could do to the marine reserve at Poor Knights.

Biosecurity New Zealand says it’s researching caulerpa’s impacts and sending a handful of experts and mana whenua to Newport to learn more about the vacuum dredge used there. In the meantime, communities are stepping up. Thorburn’s group got hold of a remotely operated vehicle—a kind of underwater drone—and is surveying the main anchorages on Waiheke three times a year, in the hope members will catch any infestations early enough to destroy them.