The Southland Regional Council has set up a controlled area in part of Breaksea Sound in an attempt to stop an invasive seaweed spreading in Fiordland waters.
The marine pest, known as Undaria or Asian kelp, was first found in Sunday Cove in the Breaksea Sound in 2010, and officials have been trying to get rid of it ever since.
But a survey last year found the marine pest had spread, infesting the nearby Beach Harbour and John Islands area.
The seaweed, which can attach itself to boat hulls or mooring ropes, can smother other organisms and damage eco-systems.
The council's biosecurity manager, Richard Bowman, said to control the marine pest, boats could no longer anchor in the part of the Breaksea Sound and the use of moorings was restricted to 48 hours at a time.
Mr Bowman said any dive gear used within the controlled area had to be treated or dried prior to being used again.
"We know the damage a marine pest such as Undaria could do to the ecology and economy of this area, so we need to consider every option.
"Fiordland is an incredibly precious and unique area, it is a national treasure and needs to be protected," he said.
Mr Bowman said it would be meeting with marine scientists this month to consider an intensive control programme for Undaria.