Stewart Island is known to be stoat-free with the nearest population of the predator 30kms across Foveaux Strait.
The Department of Conservation says stoats can only swim 2km and they are not known to stow away on vessels.
"This stoat has died from a head injury then mysteriously turned up on a busy walking track," Doc southern South Island operations director Aaron Fleming said.
The stoat's skull was crushed indicating it had died instantaneously from either a trap or similar blunt force prior to being found on the track.
"We need to know if we are dealing with a new significant threat to Rakiura’s native species, or if this stoat has been transported from elsewhere and planted deliberately. Both scenarios are disturbing. We are taking this seriously," Mr Fleming said.
The dead stoat has been sent to an expert to see what further information can be obtained. Samples are being sent for genetic testing to try to determine where it has come from.
Mr Fleming said it would have "devastating implications" for native wildlife if stoats took hold on the island.
"If stoats were to establish on Rakiura they would threaten efforts to create a predator-free space for kākāpō to be returned, and they would quickly decimate other native species on the island."
The Predator Free Rakiura project is currently working on the removal of Rakiura’s existing pest species – rats, possums, feral cats and hedgehogs – in one of the largest and most complex predator eradication projects ever attempted on an inhabited island.
Introducing pests such as stoats constitutes an offence in New Zealand, with serious consequences and penalties including imprisonment of up to five years and significant fines.
Anyone with information about the dead stoat's origins is encouraged to contact DOC 0800 DOC HOT (0800 362 468).
