The reserve this year has 30 nests and about 100 of the yellow-eyed penguins. About 600 live in the South Island, from an estimated total of 3000 of the Megadyptes antipodes, which live only in New Zealand and its offshore islands.
''I was stunned to come across this bird and cat,'' Mr Johnson said. Later that evening he spoke to Rosalie Goldsworthy of the Katiki Point Penguin Trust, which looks after the reserve. She assured him the cat must be feral, and she would be putting out a trap to catch it. No domesticated cats lived in the area. He suggested shooting it would be more successful as cats were very difficult to trap.
''Perhaps Gareth Morgan made a good point about cats a few weeks ago,'' Mr Johnson said, referring to the anti-cat campaigner's opposition to feral cats and his call for a cat-free New Zealand.
Mrs Goldsworthy was not sure if the cat photographed by Mr Johnson had been caught, but she said about 20 feral cats were captured in traps every year at what was one of the biggest yellow-eyed penguin colonies in New Zealand. Shooting was not allowed on the reserve, except under extraordinary circumstances.
In her 11 years looking after the Moeraki lighthouse reserve no yellow-eyed penguins had been lost to feral cats. One little blue penguin had been killed by a large cat some years ago and a licensed pest control agent had been called in to kill that cat.
Also, the penguins nested in boxes that gave them protection on three sides, with the parent guarding the opening.
''Feral cats are an ongoing problem, but we have them under control through trapping,'' she said.
Caught feral cats were humanely killed.