Rat tracks have been seen on Rangitoto Island in the Hauraki Gulf, sparking a search and trapping operation by the Department of Conservation.
The department has been treating Rangitoto and Motutapu islands as pest-free following an eradication programme in 2009.
DoC says the tracks were found in a routine check of tunnels near the wharf and the rat probably got on the island from a passenger boat such as a ferry or kayak.
The department has set 175 traps and says it will comb the area with dogs. However, it is not sure how may rats there may now be.
It says an individual rat will not do a huge amount of damage but rats are very destructive in forests so it is keen to ensure that no population establishes.