Boaties and cray fishers are being asked for their help to stop a highly invasive kelp spreading around Northland harbours.

Undaria pinnatifida has made itself at home in Rangaunu and Houhora harbours.

The seaweed, which resembles some native kelps, is also known as Japanese or Asian kelp and is farmed in parts of Asia and Russia where it is used as a vegetable and in skin products.

Nicknamed ''the gorse of the seas'', it is established in Fiordland where it is seen as a threat to the unique marine environment. It is believed to have settled in several parts of New Zealand's coast over the past 20 years.

Don McKenzie, biosecurity manager for the Northland Regional Council (NRC), said it was first detected in Rangaunu and Houhora harbours about three years ago.

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"It's a serious biosecurity issue because it can be hard to distinguish from native kelps, is fast-growing and in other parts of New Zealand has already impacted on the aquaculture industry, fouling mussel lines and other equipment."

He said the kelp's rapid growth rate — up to 1cm a day — means that as a marine pest it's as bad as Mediterranean fanworm, another unwanted marine pest in Whangarei Harbour.

Undaria's presence has prompted a renewed call by the regional council for boaties to keep their hulls clean, and for cray fishers to check their pots and other equipment.

Mr McKenzie said the challenge is now to contain the ''opportunistic'' kelp and keep it out of other harbours in the region.

That will be no easy task because vessels based in the Bay of Islands often travel to and from the Houhora and Rangaunu harbours, he said.

"Some of these vessels could well travel to the abundant waters around the Three Kings Islands (Manawatawhi), heightening the biosecurity risk to this high value marine ecosystem."

A soft cloth clean would be enough to clean vessels of a slime layer.

"Anything more than that – such as evidence of actual undaria growth – should be reported to the regional council. We'll work alongside vessel owners to deal with this problem on a case-by-case basis."

The council's inspection of a planned 2000 boat hulls in Northland over summer is under way. The council is also in the process of adopting a new Marine Pathway Plan that introduces new rules on hull-fouling.