All traps designed to trap stoats have been rebaited after a dead stoat was found in a trap on pest-free Motutapu Island.

A stoat has been trapped on pest-free Motutapu Island in the Hauraki Gulf.

A DOC ranger discovered the animal during a routine trap check near Emu Point on the Motutapu side of Islington Bay, which connects Motutapu to Rangitoto.

DOC spokesperson Keith Gell said stoats were "public enemy number one" for native birds.

"A stoat poses a massive risk to threatened and at-risk native birds on Motutapu, including kiwi, takahē, tīeke or saddleback, shore plover or tūturuatu, pāteke and kākāriki."

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He said the pests were voracious and relentless hunters eating eggs, chicks and adult native birds.

Gell was disturbed the stoat managed to travel to Motutapu but said biosecurity systems were able to catch it quickly.

The stoat, which was killed by the trap, had been sent to Auckland for DNA analysis to try and find out where it came from.

Gell believed it could have hidden on a vessel that travelled to Motutapu or sailed near the island, or it could have even swum to the island.

"This reinforces the need for boat owners 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," Gell said.

Both Fullers and 360 Discovery who run ferries to Motutapu and Rangitoto have warrants that show they meet biosecurity standards.

After finding the dead stoat all traps on the island had been rebaited with fresh rabbit meat and fresh eggs.