A notorious marine pest has been found growing on the Bay of Islands seabed for the first time.

Last week's find of a single Mediterranean fanworm on a mooring block opposite Opua Marina sparked a biosecurity operation with two teams of divers searching the surrounding area.

The divers found another dozen fanworms at three locations in several hundred square metres around the initial find.

Northland Regional Council marine biosecurity manager Sophia Clark said the pest was found by mooring contractors on Tuesday and confirmed as fanworm yesterday. Diver contractors had searched the area from Thursday to Sunday.

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The marine invader — which can form dense beds smothering native species and weighing down marine structures — was likely to have come from an unidentified, and long departed, infected vessel.

Marina and mooring owners in the area had been notified, Clark said.

''We're taking this very seriously ... Divers will continue to search on a regular basis over the next couple of months and remove any fanworm they find, but so far searches of other moorings and vessels in the wider area have been fanworm free."

Clark hoped the invader had been caught early enough to stop it becoming established.

A few of the individuals found were large enough to have reproduced. Testing of samples sent to Niwa's Marine Invasive Taxonomic Service would determine whether that had occurred.

Eradication of fanworm in newly infested areas was possible but only if they were caught early.

Clark urged boaties to make sure their vessels and equipment were clean and free from fouling to avoid bringing marine pests into, or around, Northland.

It is an offence to bring a boat into Northland, or from one harbour to another, with more than ''light fouling'' — defined as a slime layer and/or barnacles and up to 5 per cent macrofouling.

Officials were grateful to the contractors who spotted the pest and promptly reported it.

The Mediterranean fanworm is already established in Auckland's Waitemata Harbour and in parts of Whangārei Harbour, where it was first detected in 2012. Marsden Cove Marina in particular has been locked in a costly battle to get rid of the invader.

Tutukaka Harbour is still about a year away from being declared fanworm-free after an incursion in 2015. In the same year the pest was found on a fishing boat in Whangaroa Harbour but it had not spread to the seabed or wharf.

Between October and May regional council contractors checked 2000 vessels in the Far North.

Fanworms were found on nine boats in the inner Bay of Islands between Russell and Opua, two in the outer Bay of Islands and two in Whangaroa. None were found in Houhora Harbour, Doubtless Bay or around Kerikeri.

Any boat owner with an infestation is issued a ''notice of direction'' under the Biosecurity Act limiting the vessel's movements and usually ordering a haul-out and clean.

■ Call 0800 002 004 or email marinebiosecurity@nrc.govt.nz straight away if you think you've found a marine pest.