''We are cautious. We don't want to get overconfident,'' rat eradication specialist and incursion programme leader Peter McClelland said yesterday The 266ha island in Paterson Inlet, Stewart Island, is home to many endangered birds.
It is considered predator free, although on average, one rat a year turns up there and is caught and killed.
Mr McClelland said the most recent unwelcome guest - smart enough to eat bait laid outside traps but not to enter those traps - was recorded on video in the Post Office Cove area the week before last, eating poisoned bait on two nights.
The last time he was seen was on Monday last week when he ''gave a little show and tell for us''.
Mr McCelland said rat dropping with traces of blue-green dye were found last week, indicating the rat had ingested the bait.
But despite searches by Sandy King and her rat detection dog Gadget, who, he said, ''knew the Post Office Cove area very well'', the rat had not been found dead or alive.
It was possible it had gone into an underground burrow to die, or had been eaten by a weka, he said.
''We're disappointed we haven't found a body ... The trail has gone cold.''
Mr McClelland said monitoring would continue for at least another month, and early next month another rat detection dog and handler would search the island with Ms King and Gadget.
Only then would Doc be able to say whether Ulva was predator-free again, he said.
