Peter de Graaf

An invasive seaweed described as the world's worst marine pest has been found at Leigh, a short distance from the famous Goat Island Marine Reserve.

The roughly one-square-metre clump of exotic caulerpa was found in Omaha Cove by divers working for Auckland Council on Friday.

Biosecurity New Zealand said the find had not yet been positively identified - but it was highly likely to be the fast-growing pest.

Its readiness and response director John Walsh said if confirmed, the patch would be treated as soon as possible by council and mana whenua.

Until more was known he could not speculate on where the caulerpa came from or what action would be taken next, he said.

The seaweed pest was found in shallow water during routine surveillance of high-risk areas.

Walsh said since exotic caulerpa was first found in New Zealand, at Aotea Great Barrier Island in 2021, $11 million had been allocated to a coordinated response with partners such as mana whenua, regional councils and the Department of Conservation.

That response sought to understand the pest and its distribution, prevent its spread, and explore ways of removing it, where that was possible.

A $5 million programme was currently underway at Rāwhiti, in the Bay of Islands, to fast-track the development of technology to find and remove exotic Caulerpa, he said.

RNZ understands the suspected caulerpa was found near boat moorings - but Omaha Cove also has a wharf used by commercial fishing boats, including those permitted to work in the controlled areas around Aotea Great Barrier where the pest is rife.

There was as yet, however, no way of knowing how the seaweed pest reached the cove.

Omaha Cove, at Leigh, was a short distance south of Te Hawere-a-Maki/Goat Island Marine Reserve, one of New Zealand's oldest and best known marine protected areas.

Leigh was also the location of Auckland University's marine laboratory.

The beachside destination of Omaha, known for its top-end holiday homes, was a short distance further south.

Since its discovery at Aotea Great Barrier Island, exotic caulerpa has been identified in Te Rāwhiti Inlet in the Bay of Islands, Ahuahu Great Mercury Island, Kawau Island, Waiheke Island, Mokohinau Islands, Rakino Island and Fantail Bay on the Coromandel Peninsula.

Exotic caulerpa was regarded as a serious threat to biodiversity, kai moana, and recreational and commercial fishing, due to its ability to spread rapidly over large areas of seabed and smother other species.

This story was orginally published by RNZ