Department of Conservation Rakiura operations manager Jennifer Ross said Ulva Island was vulnerable to reinvasion because of its close proximity to the mainland.
Normally the island’s strong trapping network and monitoring programme were enough to get on top of any incursion.
"However, despite our efforts the rats managed to establish a population on the island over the following few months.
"We’ve been working closely with the Rakiura community on the eradication plan and aim to undertake the operation in the first good weather window."
The island had been established as a pest-free sanctuary after an intensive rat eradication programme was launched in 1992 and declared rat-free in 1997, but from 2021, numbers had begun to fluctuate again.
Doc held a public consultation meeting with Stewart Island/Rakiura residents last week to explain cereal baits, containing the anticoagulant brodifacoum, would be dispersed in two drops by air during the first clear weather window from July 17. A second application would follow about 10 days later.
Stewart Island resident Margaret Hopkins said the general feeling on Stewart Island was people wanted them to "hurry up and do it".
"It’s been so disappointing it’s taken this long to actually get some action. We want to get it back to predator-free."
It was not uncommon for the odd rat to make it on to the island.
"But this time there was more than one rat and it took a while to realise the extent of the numbers that were there," she said.
She believed rat numbers were unlikely to be large enough to impact native fauna because there was still plenty of food in the bush.
"It takes a while before you see actual impacts on bird life, but also on invertebrates and things like that. It all can get affected when the rat numbers get high.
"But I don’t think the rat numbers are in plague proportion. They just know that they are there and it’s quite difficult to get rid of them."
Ulva Island, considered to be the jewel of Rakiura National Park, is a small 267ha island with an 11km coastline inside Stewart Island/Rakiura’s Paterson Inlet.
Ulva Island/Te Wharawhara had been named one of the few pest-free open sanctuaries in New Zealand.
Access to Ulva Island will be restricted on the aerial drop days.
Fish and shellfish harvested or caught within a 1km radius of the island should not be eaten.
By Toni McDonald
