No more exotic caulerpa has been found at north Auckland's Omaha Cove since a patch of the seaweed pest was destroyed.

The patch of caulerpa, which measured roughly one square metre, was found on 7 June during regular surveillance of high-risk areas by dive contractors working for Auckland Council.

Exotic caulerpa is an invasive species that spreads rapidly across the seabed, smothering native seaweeds and shellfish beds.

It was the first time the marine pest had been found close to mainland Auckland, though there are known infestations at Aotea / Great Barrier Island, Ōmakiwi Cove in the Bay of Islands, and various locations in the Hauraki Gulf and off Coromandel Peninsula.

The seaweed found at Omaha Cove, not far from Te Hāwere-a-Maki/Goat Island Marine Reserve, was confirmed as caulerpa by Niwa experts, then destroyed on 17 June by Auckland University divers who covered the patch and dosed it with chlorine.

Liz Brooks, of Auckland Council, said the treatment had eliminated all viable exotic caulerpa in the patch.

Further searches in Omaha Cove, Leigh Harbour and neighbouring areas had not revealed any further exotic caulerpa to date.

Brooks said ongoing surveillance in the cove, along with monitoring of the treated patch, was planned.

The council would continue to work with Ngāti Manuhiri, Auckland University's Leigh Marine Laboratory, and the local community to keep an eye out for caulerpa in the area.

Divers use suction hoses to remove the treated caulerpa. Photo: Supplied/Nick Shears

Meanwhile, Ngāti Manuhiri Settlement Trust chief executive Nicola MacDonald said the elimination was an example of a successful partnership between mana whenua and local government.

"Time is of the essence when dealing with caulerpa," MacDonald said.

"Our kaitiaki were in the water straight away to assess the severity of the issue and implement an immediate treatment plan in partnership with Te Kaunihera o Tamaki Mākaurau (Auckland Council) and University of Auckland Professor Dr Nick Shears."

The caulerpa was confirmed as dead when the cover was removed from the patch on Tuesday, she said.

MacDonald said although the patch was small, it was the first time a caulerpa infestation had been eliminated.

It demonstrated the importance of a fast and aggressive response to achieving the settlement trust's aim of completely eradicating caulerpa from Te Moananui-ā-Toi/the Hauraki Gulf.

The patch of caulerpa after chlorine treatment. Photo: Supplied/Nick Shears

"We've seen what happens when it's left unchecked for too long, such as the infestation at Aotea Great Barrier Island discovered in 2021, and subsequent discoveries at Kawau Island, Waiheke Island, and Mokohinau Islands, which are proving incredibly tough to eradicate."

Trust kaitiaki would continue to monitor the area, she said.

Brooks said everyone could play a part in stopping the spread of exotic caulerpa by staying away from known infestations.

Caulerpa brought up on anchors or hooks should be bagged and binned, while anyone who found the invasive seaweed on the beach should "pin, pic and phone" (note the location, take a photo, and report to the Ministry for Primary Industries on 0800 80 99 66 or online).

This story was originally published by RNZ