The discovery of a highly invasive marine pest on a boat moored in a pristine Northland harbour is a wake-up call for the region, a biosecurity expert says.

The discovery of up to 300 Mediterranean fanworms on a fishing boat hull in Whangaroa Harbour last month has already prompted a tightening up of marina rules around the region.

A routine inspection of Whangaroa Harbour on August 19 found the fishing vessel Catherine II, tied up at Clansman Wharf, was infested with fanworms. The creature is a serious threat to native species and could wipe out Whangaroa's oyster industry.

Auckland is so badly infested the Ministry of Primary Industries and local authorities have given up trying to eradicate it. Marsden Cove Marina in Whangarei Harbour is locked in a costly battle with the pest, but the Far North had previously been fanworm-free.

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Divers removed the fanworms from the hull of Catherine II on August 21 and the boat was hauled out for cleaning at Opua on Monday.

The boat had been anti-fouled just a few weeks before it visited Auckland late last year, when it is likely to have become infested.

Irene Middleton, aquatic biosecurity officer for the Northland Regional Council, said the incident was a wake-up call.

"You can't just rely on anti-foul. You have to know what's on your hull and keep checking it, and you have to know the requirements of the places you're going to."

Diver Corey Dalley, of Kaeo, with fanworms removed from the hull of the infested fishing boat. Photo / Peter de Graaf
Diver Corey Dalley, of Kaeo, with fanworms removed from the hull of the infested fishing boat. Photo / Peter de Graaf

All marinas from Marsden Cove to Whangaroa were now insisting that arriving boats either prove they had had their hulls inspected in the previous month or had anti-foul applied within six months.

The council was spreading the message via boating magazines, websites and boat shows.

Waikato and the Bay of Plenty were bringing in similar restrictions.

Meanwhile, it appeared Whangaroa had "dodged a bullet" because the fanworms found on Catherine II were thought to be too young to have spawned into the harbour.

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Tests in the council lab had found they were immature and not yet capable of reproducing. Samples had been sent to Niwa in Wellington for further testing.

It was thought fanworms could start reproducing once they reached about 10cm in length. Those found at Whangaroa were right on the cusp.

Meanwhile, council surveillance of Northland harbours was continuing with Mangonui and Houhora due to be inspected in the next two weeks.

Divers would return to Whangaroa in six months' time.

A few young fanworms were found in Tutukaka Marina earlier this year, but were not thought to have spread further.