Biosecurity staff with sniffer dogs have been scouring Motutapu Island in the Hauraki Gulf looking for a black cat spotted on the bird sanctuary at the weekend.
The Department of Conservation (DoC) is concerned about the damage even a single cat could do on the island, which was declared pest-free in August 2011 and is now home to Coromandel brown kiwi, takahe and saddleback.
Three participants in cross-country sporting event the Dual reported seeing a cat as they ran on Saturday.
Department of Conservation spokesman Nick Hirst said two DoC biosecurity staff with sniffer dogs had been deployed to look for any sign of a feline and had set traps in the area.
There had been no sign of a cat at this stage and the people who reported it could have been mistaken.
"We get things like this on a regular basis, we check them out, but most of them come to nothing."
If there was a cat it could be devastating to the island's birds.
"A bird that lays its eggs on the ground is vulnerable, definitely," Mr Hirst said.