A stoat seen on predator-free Kapiti Island probably swam the 5km to get there, the Department of Conservation says.
A contractor spotted the pest last Thursday. They are known to swim distances of up to 4km.
The discovery worries the department because of the island's endangered native bird species, including kiwi, kokako and takahe.
The stoat was spotted by a contractor last Thursday and the Department of Conservation believes it swam the five kilometres to the island.
The department has urgently sent extra staff to the island to trap the pest with fresh rabbit meat. If that does not work, it will bring in a dog to track the stoat.
Kapiti Island, near Wellington, has been free of predators for more than a decade.
"Swimming seems the obvious reason - they would tend not to stow away in someone's bag who was visiting for the day," says the department's Kapiti area manager Rob Stone.
"We're surprised that it's managed to swim there - the water's cold and it's quite a long way."
The chairman of environmental group Kapiti Island Watching Interest, Hugh Barr, says the stoat's appearance shows the island is still vulnerable to predators.
"Island sanctuaries are a marvellous idea, but you really do need continuing vigilance all the time."
Mr Barr says any stoat sighting is a worry, but particularly so if it is a female because the animal could be pregnant.
The department says it is the only credible sighting of about a dozen reported since the island became predator-free.
