Conservation workers are on the hunt for a rat on Tiritiri Matangi Island.
The Hauraki Gulf sanctuary has been free of animal pests, including mice, since 1993.
However, two monitoring cards inside tunnels have shown a rat went through them sometime between New Year's Eve and Sunday.
Two pest detection dogs and their handlers have been searching for the rodent since Monday.
The Department of Conservation has also assembled a team and deployed more than 50 additional traps and at least 60 extra tracking tunnels.