Northlanders are being urged to report a cat-sized predator capable of killing even adult kiwi.
Ferrets are uncommon in Northland but recent sightings in the Bay of Islands have conservationists on alert.
Like stoats and weasels, ferrets are members of the mustelid family introduced to New Zealand in the 1880s in a failed attempt to control rabbits.
Unlike other mustelids, however, ferrets — which grow to about 50cm long and weigh up to 1.5kg — are big enough to take on an adult kiwi.
Kiwi Coast's Mid North coordinator Andrew Mentor said recent ferret sightings in Waipapa and Puketotara had conservation groups concerned.
Puketotara, west of Kerikeri, had high numbers of kiwi and Waipapa had a small remnant population.
In areas without intensive pest control stoats killed 95 per cent of kiwi chicks but adult kiwi were big enough to fend them off. Ferrets were a different story, however.
''We're chasing a few now in the Mid North where we've had a couple of separate sightings. We're trying to get them with traps, if that fails we'll bring in conservation dogs.''
He urged anyone who spotted a ferret to call the Northland Regional Council environmental hotline on 0800 504 639 or the Department of Conservation hotline on 0800 362 468 (0800 DOC HOT).
During the ferret fur boom of the 1980s ferret farms sprang up around Northland, including in Waipapa, Mentor said.
When the industry collapsed many of the animals were released into the wild with devastating effects for wildlife.
Ferrets were banned as pets in New Zealand in 2002 after boaties took their pet ferrets for a walk on Great Barrier Island, a nature sanctuary.
They were originally bred from wild European polecats for rabbit hunting.
Ferret-legging was an endurance test once practised by Yorkshire coal miners, and the winner was whoever managed to endure a ferret in their trousers for the longest time. The ''sport'' had a brief resurgence in the 1970s.
Ferrets are mustelids, like stoats and weasels. They are bigger than stoats and weasels and are about the same size as a small cat. In other parts of the world ferrets are used to hunt rabbits and are kept as pets.
The ferrets in New Zealand are ferret-polecat hybrids. Tame ferrets were bred with polecats during the voyage to New Zealand so they would be better at surviving in the wild. Unfortunately it worked. Now New Zealand has the world's largest population of wild ferret-polecat hybrids.
The first five ferrets were brought to New Zealand in 1879 to get rid of rabbits.