Invasive Caulerpa seaweed has spread to Whangārei Harbour for the first time.
The ‘significant’ new find at the harbour entrance is horrifying the local community.
About a square metre of the seaweed has been found in Whangārei Heads’ Smugglers Bay, on the north side of the harbour entrance.
“I’m horrified to hear about that. We have to make sure it hasn’t spread any further,” Whangārei Heads Citizens Association vice president Jan Boyes said.
“I’m fearful it might have got further into the harbour.”
The patch of caulerpa was found on Thursday June 4 during routine surveillance .
Biosecurity experts have since 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 the equivalent of 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 alongside the Department of Conservation.
NRC biosecurity manager marine Dr Kaeden Leonard said the council immediately responded after the significant 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 (SUBS: Friday 12 June).
Leonard confirmed the caulerpa under the mat had been killed.
He said there had been no further caulerpa found in Smugglers Bay and they were hopeful no further caulerpa was present.
There would now need to be work done on developing a plan with partners Biosecurity New Zealand and local iwi/hapū to check the wider seafloor around the area for any more of the seaweed.
NRC biosecurity and biodiversity working party chair Jack Craw said the find was most likely human assisted.
Smugglers Bay is a popular stopping off point 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.
Caulerpa now covers the equivalent of more than 1100 rugby fields, the country’s worst infestation.
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 chair 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 the whole of Whangārei Harbour, including Matakohe Limestone Island because of the impact its rapid spread had on important ecosystems.
Caulerpa has already invaded the rocky shorelines on islands in the Bay of Islands.
Stevens said she was concerned about the seaweed invading 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 make sure their boating anchors and gear were clean.
Crombie 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 great it had been found and dealt to but extra vigilance would now be required.
New Zealand’s last new caulerpa confirmation was at Little Barrier Island (Hauturu o Toi) in the outer Hauraki Gulf, roughly halfway between Auckland and Whangārei, in May 2025.
