David Chadfield of Chaddy's Charters said he's found dead penguins and piles of feathers as families of feral cats close in on Port Taranaki's 20 nesting boxes.
"I've found heaps of penguin bodies throughout the years. Just absolutely destroyed," he said.
"I hear them at night and in the morning, there's a massive amount of feathers everywhere."
Despite the port's efforts to reduce the feral cat population, Chadfield said it was a persistent issue that was worsening.
"I've seen cars tipping out bags of kittens in the area," he said.
"It happens all around the port.
"These people think it's not a real problem to do it, but it becomes a real problem for us."
Chadfield, who is well known as Chaddy, said he was concerned the feral cats would kill the entire population of little blue penguins.
"Cats are predators. Penguins are pretty good at defending themselves, but the cats will win in the end I'd say."
Chaddy isn't the only port person to spot the felines.
Port environmental manager Bridget Harrison said it has been a ongoing issue.
"We take our responsibility to control vermin seriously, to keep our penguin population safe," she said.
"The penguins are nesting at the moment, so it is a crucial period."
Though the port has a programme to catch cats, Harrison said they planned to increase the number of traps.
The traps, which are wire cages with a piece of meat in the back, attract the cats and an automatic door shuts behind them.
The cats are then taken to either the SPCA or the Department of Conservation (DOC), where they are scanned for microchip IDs.
Microchipped cats are returned to the rightful owner, while non-microchipped cats are euthanised.
The growing concern for the penguin population would see traps installed on Ocean View Parade and Breakwater Bay.
Though the port's increased efforts would be a one-off operation, Harrison said it would be repeated if the community felt feral cats were a continuous threat to the little blue birds.