Mice have been found on a supposedly predator-free island in the Marlborough Sounds.
The Department of Conservation (DoC) says the mice were discovered on Maud Island in searches by a rodent detection dog and trapping.
DoC says it is yet to determine the number of rodents or how long they have been on the island sanctuary for native insects and lizards.
It says an eradication programme has to be held off until next winter when food for mice is scarce and they are more likely to eat the poison baits.
DoC Sounds Conservation Services Manager Roy Grose says the department is committed to ensuring the island is restored to its predator-free status.
He says staff are particularly worried about Maud Island frogs and Stephen's Island striped gecko.
Options include moving the gecko into a safe enclosure on the island and, as a precaution, moving some frogs to set up an additional population on another predator-free island or at a wildlife centre.
