A rat appears to have made its way onto a predator-free island, off the coast of Stewart Island, home to the endangered kākāpō.
Department of Conservation Murihiku operations manager Tony Preston, said a dropping thought to be from a Norway rat has been found on the Whenua Hou, about 3km off Stewart Island / Rakiura.
"[Rats] can swim up to one kilometre. A Norway rat has made it to Whenua Hou way back in 1984 - there was a Norway rat caught in a possum trap there."
Though the island is outside their swimming distance, rats could "hitchhike" there on a log or a fishing boat.
"They breed really quickly ... two individual rats can be 15,000 by the end of the year, so we're treating this really seriously and want to get on top of it really quickly."
Mr Preston said the dropping had been sent for DNA analysis and results expected next week would confirm whether it was from a rat.
The island was regularly checked using a detector dog - the last time was in June - so if it was rat incursion it would have been relatively recent, he said.
Kākāpō are not expected to breed again on the island until 2019.
"That's a bit of a blessing in disguise because chicks would be vulnerable to predation from rats but the adults are generally large enough to cope with a rat."
The kākāpō is one of the rarest parrots in the world with fewer than 160 remaining. There are kākāpō breeding populations on two other predator-free islands, Anchor Island in southwest Fiordland and Hauturu o Toi/Little Barrier Island in the Hauraki Gulf.