Stoats are blamed for killing up to 95 per cent of kiwi chicks in areas without predator control, but an even more serious threat to the national bird has been identified in the Bay of Islands.

Ferrets, which can grow to 50cm long and weigh 1.5kg, are uncommon in Northland, but recent sightings at Waipapa and Puketotara have rung alarm bells for conservation groups, the species posing an even greater threat than their smaller cousins.

Like stoats and weasels, ferrets, fellow members of the mustelid family, were introduced to New Zealand in the 1880s in a failed attempt to control rabbits. Unlike other mustelids, however, they are big enough to take on adult kiwi.

Kiwi Coast's Mid North co-ordinator Andrew Mentor said the recent ferret sightings had conservation groups concerned. Puketotara, west of Kerikeri, had high numbers of kiwi, while Waipapa had a small remnant population.

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"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 said.

Mr Mentor urged anyone who spotted a ferret to call the Northland Regional Council environmental hotline (0800 504-639) or the Department of Conservation hotline (0800 362-468 — 0800 DOC HOT).

Ferret farms had sprung up around Northland, including in Waipapa, during the ferret fur boom of the 1980s, he added, and many of the animals had been released into the wild when the industry collapsed, with devastating effects for wildlife.

Ferrets were banned as pets in New Zealand in 2002.