The Reefton sighting was one of 11 such sightings submitted, but the others - from the Marlborough Sounds, northwest Nelson, the West Coast and the Catlins, between 1990 and 2008 - were considered to be only only "possible" or "probable".
An expert panel convened to manage the New Zealand Threat Classification System (NZTCS) earlier this month changed the South Island kokako's classification from "extinct"' to "data deficient", based on the 11 claimed sightings.
Forest and Bird advocacy manager Kevin Hackwell said that while it couldn't be confirmed the South Island kokako was still alive, "this is the best sign yet that it is".
"Sadly, when the claimed Reefton sighting was made, the area was subject to intensive pest control. But that pest control stopped several years ago," he said.
"Because of the reclassification, there needs to be more pest control work in the South Island than ever before.
"If they are still out there, the South Island kokako will just be hanging on, and their biggest threats will be rats, stoats and possums.
"New Zealand is thought to have lost over 50 bird species. If just one of those extinctions turns out not to have happened, it would be incredibly good news."
Forest and Bird member Alec Milne, who claims to have heard and seen South Island kokako, and runs a project to find evidence they still exist, described the Ornithological Society's decision as "wonderful".
The birds have - or had - orange wattles on their faces. The endangered North Island kokako has blue wattles.