Tests are being carried out to determine if seaweed hauled up on a yacht's anchor in the Bay of Islands before Christmas signals a new marine biohazard.
A Ministry for Primary Industries' (MPI) algal taxonomist has given a preliminary identification of cladophora ruchingeri, from a family of seaweed also known as mermaid's hair.
The discovery at Lagoon Bay, Roberton Island, raises fears there is an established new biofouler that could attach to boats and structures, and be spread.
Northern Regional Council biosecurity manager Don McKenzie said the council would determine its response when it had an update from MPI on the algae's taxonomy.
Russell yacht tourism operator Stephen Western said he believed that if the weed took hold it could devastate scallop beds.
Mermaid's hair must have been brought in to the area on a boat.
"Boaties should be advised to clean it off their anchors before changing bays," Mr Western said.
He sent samples of the Lagoon Bay find to MPI.
The only other place cladophora ruchingeri is known to live in New Zealand is the Marlborough Sounds, where a few years ago it was found attached to ropes at green-lipped mussel farms.
That discovery was said to be the first in the Southern Hemisphere and Pacific region, although worldwide there are similar species, including freshwater types.
Further MPI tests, including DNA typing, would determine if the Bay of Islands incursion is the same or another species.
"[This sample], if confirmed, would represent a considerable range extension for this known biofouler," a spokeswoman said.
"MPI believes this is the same genus as that found in the top of the South Island but we cannot say definitely what the species identification is until molecular tests are completed," MPI told the Northern Advocate yesterday.
But whether there are two near identical types from both the Pacific and Northern Hemisphere, or their only difference is down to a spelling error, might also need to be determined.
Mr Western has heard from marine biologist, Don McClary, who was visiting Bay of Islands around the time the weed was found, and had seen something similar - but smaller in quantity - at Oke Bay and Awaawarua Bay.
He believed the local sample came from the tropical Pacific.
"This and the location strongly suggests it was brought in on a cruising boat rather than from down south," Mr McClary said.
He said there is both a cladophora rechingeri and a cladophora ruchingeri listed by scientific research.
The latter is said to be from the tropical Pacific, the former - the type on Marlborough Sounds mussel farms - said to originate from the Northern Hemisphere.
"It may just be a name mix up, but they may be different," Mr McClary said.
The DNA test would determine that, he said.
Cladophora are often recognised by branched, long filaments that swirl in the wave action — such as that which clings to the Punakaiki Pancake Rocks on the West Coast.