It is "highly unusual" up to three stoats may be plaguing Motutapu and Rangitoto islands in Auckland’s Hauraki Gulf.
The Department of Conservation told 1 NEWS the islands usually only get one stoat every three to four years.
In the last 12 months, up to five stoats have made their way to the islands, with two being trapped.
It has led DOC to describe the number of stoats detected on the islands in this space of time as "highly unusual".
The presence of a stoat was first detected on the islands in May last year. It was believed to have island-hopped from nearby Motukorea/Browns Island.
The male stoat, weighing close to 400 grams, was trapped months later in September 2020 and cost DOC and Auckland Council about $86,000 to do so, Stuff reported.
Another male stoat was trapped in January this year.
However, the three other stoats, described as "very wary", continue to plague the islands.
To date, DOC has spent $241,178 trying to catch them.
The stoats have forced Motutapu Island’s population of 10 tūturuatu (shore plover) to be evacuated to a sanctuary and wildlife centre after three were found dead.
The tūturuatu is one of the world’s rarest shore birds. With a population of about 250 birds, the survival of the endangered species relies on island biosecurity, breeding in captivity and predator-free islands.
The three found dead had been killed at night while incubating their eggs, causing a serious setback for the population which has struggled to become established on the island.
Kat Lane, DOC’s operations manager, told 1 NEWS four tūturuatu had recently been returned to the island.
Any more tūturuatu returned to the island would be closely monitored, she said, which included the use of radio transmitters.
"There are limited safe habitats for these small shorebirds, and existing islands are already at capacity. The birds will attempt to return to their home on Motutapu if they were released elsewhere," she explained.
It is also believed a non-native sparrow, whose remains were found by a mustelid (stoats, ferrets, weasles) dog, was most likely killed by one of the stoats.
Lane said analyses of stoat faeces had confirmed the presence of the three stoats, with DOC believing all had arrived independently on the islands.
"One male stoat was identified and there are two partial profiles meaning there could be up to two additional individuals of indeterminate sex.
"As there is no evidence of breeding on the island last spring (in the form of easily trapped juvenile stoats in autumn), we have an indicator that all the stoats that arrived on the island in 2020 were male."
Lane said nearly 600 traps were in place across both islands to try and trap the stoats. A further 20 traps scented with female stoat lure have been deployed in locations where fresh stoat sign has been found by a mustelid detection dog.
DOC is also calling for volunteers to review trail camera footage from Motutapu in an effort to find the stoats.
The stoats also forced iwi Ngāi Tai ki Tāmaki to place a rāhui on Motutapu Island in May. It remains in place.
While it does not affect Fullers ferry services to Rangitoto, access to Motutapu is restricted and bookings for the campsite on the island are unavailable while the rāhui is in place.
"Eliminating a very small number of stoats roaming over two large islands with abundant bird life available for the stoats to eat is much harder than killing the same number of stoats on the mainland," Lane said.
"Our trapping and luring is being fine-tuned all the time in response to the challenge of getting them to engage with our tools.
"We now have extended the number of trap types, trap-housing and lures deployed to encourage the remaining stoat or stoats to enter a trap."
Visitors to pest free islands should: