When new individuals from a pest species are detected following eradication, identifying whether the new individuals are survivors from the eradication attempt, or reinvaders from another population, is important for management practices. Pearl Island (512 ha) in New Zealand was the first island in the world on which simultaneous eradication of all three invasive rat species was attempted. Rats were detected again 9 months after the eradication operation. We use genetic assignment methods to discriminate between survivor and reinvader hypotheses. All rats found on Pearl Island after eradication were likely to be reinvaders from an adjacent population on much larger Stewart Island (174,600 ha), suggesting that rats were swimming to the island at a rate much greater than anticipated, but that the original eradication was successful. Adequate genetic signal was obtained from opportunistically collected samples, making the method feasible for conservation managers with limited time and resources.