The reward for spotting the long-believed extinct South Island Kōkako doubled earlier this month to $10,000.

Since a $5,000 reward was offered in January, there have been around 50 reports of possible encounters with the bird – nicknamed the 'grey ghost' for its elusiveness.

Sightings and 'hearings' of the bird have come in from the Marlborough Sounds, the Heaphy Track area, the West Coast (particularly the Grey Valley), South Westland, Fiordland and the Catlins, says Inger Perkins, general manager of the South Island Kōkako Trust.

"Wherever people are going in those areas, wherever they're getting outdoors, we want them to be listening out and looking out for it."

It is photographic proof that will confirm the bird's existence, she says.

"We want everybody to be on the look-out and be prepared, hopefully with a camera. We really need something visual that's going to confirm it for us and confirm it for the independent experts, as well … We are convinced it is out there."

The last accepted sighting of the orange-wattled kōkako – a separate species from the endangered blue-wattled North Island kōkako – was in Mount Aspiring National Park in 1967.

It was declared extinct in 2007, but in 2013 its status was upgraded to 'data deficient'.