The Department of Conservation is scrambling to find a rogue rat on Tiritiri Matangi Island.
Footprints were found on two of the monitoring cards inside tracking tunnels, small boxes with ink on the bottom, on the inner Hauraki Gulf sanctuary.
The cards showed a rat had been through between New Year's Eve and January 7.
Two post-detection dogs and their handlers, and four DoC workers have been deployed, armed with 50 additional traps and at least 60 extra tracking tunnels.
Auckland Inner Islands Operations Manager Keith Gell says the dogs are still completing a sweep of the island, but it is uncertain how long they'll have to keep looking.
"The operation will continue until we can be confident the island is once again free of predator-pests."
Tiritiri Matangi has been free of animal pests, including mice, since 1993.
It's home to a wide range of native birds including takahē, kiwi and kōkako. There are also tuatara, wētāpunga and five types of native lizard.
DoC says even a single rat could do significant harm to the island's relatively small populations of the animals.
Mr Gell says it is unknown how a rat got to the island but it was most likely on a visiting boat.
It is uncertain whether the rat is female, and whether it is pregnant.