The patch of caulerpa was found on June 4 during routine surveillance.
Biosecurity experts have treated the area and are hopeful it has been killed.
Caulerpa, which spreads and smothers other marine life, was first found in New Zealand at Great Barrier Island in 2021, where it now covers an area equivalent to more than 1100 rugby fields.
It was confirmed in Northland for the first time in the Bay of Islands in May 2023.
Northland Regional Council (NRC) divers found the new caulerpa patch during routine surveillance with the Department of Conservation.
Northland Regional Council divers Adi-Grace Mooar (left) and Monica Nevill-Jackson at Smugglers Bay, where a small amount of the invasive seaweed caulerpa was found.
NRC biosecurity manager marine Dr Kaeden Leonard said the council responded immediately after the find.
“That’s the basic principle of a good biosecurity response. With early detection and rapid response, you have the best chances of getting it under control,” Leonard said.
NRC divers returned to the site on June 5 to treat it with a special seafloor mat and chlorine.
The mat was placed over the caulerpa and anchored to the sea floor before being lifted about 1.30pm today.
Northland Regional Council divers placed Benthic mat (left) over a small patch of the invasive seaweed caulerpa (right) that was recently discovered on the sea floor at Smugglers Bay. Photo / NRC
Leonard confirmed the caulerpa under the mat had been killed.
He said no further caulerpa had been found in Smugglers Bay and they were hopeful none was present.
Work would now be needed with partners Biosecurity New Zealand and local iwi/hapū to check the wider seafloor for more of the seaweed.
NRC biosecurity and biodiversity working party chairman Jack Craw said the seaweed most likely arrived in the area with human assistance.
Smugglers Bay is a popular stop for boaties travelling between Auckland and the Bay of Islands. It’s about 150km by sea from Auckland and 90km from the Bay of Islands. It is also about 140km by sea from the popular boating destination of Great Barrier Island.
Craw said the Smugglers Bay caulerpa could not have come on sea currents as the seaweed did not travel long distances or survive in deep water.
Whangārei Harbour’s Matakohe Limestone Island chairwoman Pam Stevens said the find was a disaster.
“We have already got enough problems in the harbour,” Stevens said.
She said the caulerpa discovery made her concerned for all of Whangārei Harbour, including Matakohe Limestone Island, because of the impact its rapid spread had on key ecosystems.
Caulerpa has already invaded rocky shorelines on islands in the Bay of Islands.
Stevens said she was worried about the seaweed invading the nearby Whangārei Harbour marine reserve at Reotahi, where hundreds of schoolchildren were taken snorkelling each year to explore its habitats.
Leonard said the find was about a kilometre from the reserve.
Marsden Yacht and Boat Club vice-commodore Grant Crombie said it was a reminder for boaties to be responsible and ensure their anchors and gear were clean.
He likened caulerpa to cancer: “One dot on your skin and it can spread. That’s the scary part of it. If it’s been discovered at Smugglers, where else haven’t they found it yet?”
Crombie said the club would likely discuss the new caulerpa find at its committee meeting on Monday.
Meanwhile, Whangārei Cruising Club commodore Scott Gavin said nobody wanted caulerpa to smother the Whangārei Harbour seafloor.
He said it was good it had been found and dealt with, but extra vigilance would now be required.
New Zealand’s last confirmed new caulerpa detection was at Little Barrier Island (Hauturu o Toi) in the outer Hauraki Gulf, about halfway between Auckland and Whangārei, in May 2025.
■ LDR is local body journalism co-funded by RNZ and NZ On Air.