The Department of Conservation (DOC) has been alerted to a possible sighting of a stoat on pest-free Motuihe Island in the Hauraki Gulf.

DOC Auckland Inner Islands Operations Manager Keith Gell said a member of the public believed they had seen a stoat on the island.

"We're taking this reported sighting very seriously," Gell said.

All the traps on Motuihe had been baited with a combination of fresh rabbit meat and eggs, which is proven to lure stoats into traps, following the reported sighting.

"Two weeks ago, we trapped and killed a stoat on Motutapu, at Islington Bay, which connects Motutapu and Rangitoto islands," Gell said.

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"Since a DOC ranger found the dead stoat during a routine biosecurity check, we've found no evidence of a live stoat on Motutapu or Rangitoto.

"Motutapu is just 1.8 kilometres away from Motuihe."

DOC was working to determine if there was a live stoat on Motutapu, Rangitoto or Motuihe, he said.

DNA analysis, on the stoat trapped on Motutapu, had shown that it came from the Auckland mainland.

But the analysis was unable to identify where on the Auckland mainland it came from.

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

"Or it may have swum to the island."

Boat owners need to make sure there was not a stoat, rat or mouse, stowed away on their vessel, whenever they set out to sea in the Hauraki Gulf, said Gell.

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Fullers and 360 Discovery, who run a ferry service to Motutapu and Rangitoto, had a pest free warrant from DOC.

The warrant shows that Fullers and 360 Discovery meets DOC's biosecurity standards for sailing to and past pest-free islands in the Hauraki Gulf.

"Day trippers and people staying at a campsite on a pest-free islands must remember that cats and dogs are not allowed on pest free-islands," said Gell.

"They pose a major threat to threatened and at-risk native wildlife on these islands."

It was also important people did not touch traps and other biosecurity devices on pest-free islands, he said.