The bird was discovered on Waitangi Day by Kakapo recovery ranger Chris Birmingham after hearing him boom in an area not known for any kakapo activity.
Kakapo recovery team leader Deidre Vercoe said Rangi, believed to be aged between 50 and 100, was originally from Stewart Island and was one of four to be banded and released in 1987.
"But we never heard from him again . . . until now."
Rangi had not only survived cats on Stewart Island but two aerial bait drops on Codfish Island and "also he has spent a long time by himself".
Some Department of Conservation staff suggested it was possible other birds could be located on the island, and there might be more in Fiordland, she said.
A dominant male on Stewart Island, Rangi opted to distance himself from the main population but it is hoped his rediscovery will add a much-needed genetic boost to the population of 91.
It is expected to be a bumper season for the critically-endangered kakapo, with 22 of the 38 breeding-age female kakapo having mated, including six for the first time.
Mating is expected to continue throughout February.
The current egg tally sits at 17, with four being confirmed as fertile and the rest yet to be checked.
Six other eggs have already failed, with four found to be infertile, one an early embryo death and one a late embryo death.
