A dozen Norwegian rats have been caught over the past fortnight, including a juvenile, indicating there could be a breeding population on the Paterson Inlet island, Department of Conservation Stewart Island biodiversity programme manager Brent Beaven said.
"That made us hit the panic button."
Staff hoped four rats caught over winter in the more than 150 devices on the island was the last, but the discovery of rat trails by an eagle-eyed member of the public in October raised the alarm.
While no rats were caught between August and December, a dozen caught in the new year meant "we probably had a pregnant female sitting somewhere".
The 256ha "virgin forest" island, whose rat population was removed 13 years ago, was home to endangered birds such as the mohua and saddleback.
"Saddlebacks are particularly vulnerable to rats."
Mr Beaven said those birds faced a bleak future unless rats were eradicated from the island, and he conceded some of the birds may have already died.
Each year about one rat, which had arrived either by swimming or stowing away on a boat, was caught in traps, but this was the first time one had bred on the island, he said.
Doc staff had been diverted from other tasks to check traps and tunnels, and a team of pest experts would meet to plan the eradication of rats from the island.
