The Department of Conservation is "taking seriously" a possible stoat sighting on Auckland's pest-free Motuihe Island.

Its Auckland inner islands operations manager Keith Gell said a member of the public tipped off the agency about the possible presence of the mammal.

The alert comes two weeks after the department trapped and killed a stoat on Motutapu Island, which is 1.8km from Motuihe.

A 2013 University of Waikato study found stoats can swim for more than 1.8km.

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Stoats were introduced to New Zealand in the 1800s to control rabbit populations and have since had a devastating effect on native wildlife.

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The predators, which have razor-sharp teeth, have been known to attack native birds much larger than itself, like pukeko.

Since the sighting, all traps on Motuihe have been baited with a combination of fresh rabbit meat and eggs, which is proven to attract stoats.

The stoat caught on Motutapu was found near the narrow land link at Islington Bay, which connects the island with Rangitoto Island.

DOC A DOC camera catches the moment a stoat takes down a native pukeko.

DNA analysis on that stoat showed it came from the Auckland mainland.

"The stoat may have hidden on a vessel that travelled to Motutapu or sailed near the island. Or it may have swum to the island," Gell said.

"Boat owners need to make sure there isn't a stoat, rat or mouse stowed away on their vessel, whenever they set out to sea in the Hauraki Gulf."

Domesticated animals also pose a "major threat" to wildlife and should be kept off DOC islands, Gell said.

"It's also very important that people do not touch traps and other biosecurity devices on pest-free islands."