A rat has been trapped on an island in Nelson's Abel Tasman National Park that was declared vermin-free in 2009.
The discovery of the unwanted rodent on Adele Island has sparked a search to ensure it was a lone invader.
The Department of Conservation Motueka area manager, Martin Rodd, says the male rat was found on Thursday and appears to have been dead for about two days.
He says more predator traps will be installed, monitoring will be increased and a tracking dog will comb the 87-hectare island for any more rats.
Mr Rodd says the rat could have either swum 700 metres from the mainland or hitched a ride on a boat. It could have devastated the South Island robin population, as well as other birdlife, lizards and weta.
He says it is vital that people thoroughly check kayaks and boats before visiting predator-free islands.
Monitoring systems showed no sign of rats when checked in mid-December last year, he says.