On Tuesday afternoon, the Department of Conservation discovered the invader in a trap, sparking an incursion response which includes ramped-up biosecurity checks.
Ulva Island is a popular travel destination for its untouched bush and wildlife, and has been pest-free since 1997.
Many visitors make the trip from nearby Stewart Island/Rakiura, which is less than 1km away at its closest point.
Doc Rakiura operations manager Jennifer Ross said the government agency was working to determine where the animal had likely come from.
"Rats are caught on Ulva Island from time to time as it is within easy swimming distance from the mainland for a rat.
"The island is popular with visitors and there are regular boat movements to the island which can accidentally help rats arrive on Ulva."
Some of the work being undertaken to determine the animal’s origin included genetic analysis in relation to rats caught on Ulva Island/Te Wharawhara prior to an August 2023 eradication.
It would also include genetic analysis against those on the mainland.
If more rats were found, there was risk a breeding population had formed and rats were being born on the island, she said.
If that happened, the population could grow quickly and become difficult to remove entirely.
But for now, there was no reason to panic, incursions happening once or twice a year on average due to the sanctuary’s proximity to Stewart Island/Rakiura and the volume of visitors.
This week’s discovery was the first since last August’s eradication effort, Doc having undertaken "intensive monitoring" on the island.
That included the use of rodent detection dogs, trail cameras and a network of traps.
Stewart Island/Rakiura has also been in the headlines for rodents this week, after it was revealed an image of a potential mouse had been captured on trail cameras.
Mice have been unable to establish on Stewart Island/Rakiura due to predation from rats.
Environment Southland was involved in a response to that incident because of its responsibility under its own regional pest management plan.
That responsibility did not extend to Ulva Island/Te Wharawhara, because of its status as public conservation land, Environment Southland biosecurity and biodiversity operations manager Ali Meade said.
If it was discovered rats had been purposely transported to the island, then the council would deal with the offence.
■LDR is local body journalism co-funded by RNZ and NZ On Air.
