Environment Southland, the Department of Conservation (Doc) and the Ministry for Primary Industries, with support from the Fiordland Marine Guardians, are engaged in an undaria fight.
The weed was first discovered in New Zealand in 1987 and is now found in most ports, and spreads attached to the hulls of vessels or on mooring lines and equipment.
A routine marine biosecurity inspection in early 2010 by Doc staff discovered a single mature undaria plant on a mooring rope in Sunday Cove, at Breaksea Sound, Fiordland.
An early survey found hundreds of juvenile undaria plants in the cove itself, but none elsewhere.
The discovery triggered a removal programme, due to continue until 2015.
Dive teams survey the 8ha search area three to five times a day for four days, about every four weeks.
To date, 1887 undaria plants have been removed.
About 35,000 kina were collected from nearby Fiordland waters and transferred to Sunday Cove to act as a bio-control agent in June 2011.
Kina is a native sea urchin that grazes on seaweed and the $400,000 programme has worked well, Doc response manager Derek Richards said.
The amount of undaria has reduced significantly.
In 2012, one mature plant and 142 juveniles were detected; in 2013 only three juvenile plants were found.
The response team was cautiously optimistic undaria could be eradicated from Sunday Cove. If achieved, it would be a world first from the natural seabed.
However, the ultimate success of the eradication programme and keeping undaria and other marine pests out of Fiordland for the long term will depend, largely, on the co-operation of the boating and yachting communities, Environment Southland biosecurity manager Richard Bowman said.
''We don't want to be in the business of cure. We want to be in the business of prevention.
''So we really need the support of vessel owners to ensure they don't bring undaria in from infected ports - Auckland, Marlborough, Lyttelton, Port Chalmers, Bluff, you name it,''The situation is preventable.
''There are a number of ways a boat owner can help halt the spread of undaria and other marine pests.
''It is essential they clean their hull and marine equipment before entering Fiordland and that they do not remove any seafood from Sunday Cove as it may be infected with undaria spores,'' Mr Bowman said.
