It took years to build up a colony of blue penguins on the West Coast, but it took dogs just a matter of days to kill the lot.
At least 15 birds at Cape Foulwind, near Westport, have been found dead over the past week.
It is likely the entire colony is gone.
West Coast Blue Penguin Trust ranger Reuben Lane said the dead penguins had injuries consistent with a dog attack.
"They all had classic puncture wounds to the head, neck, and upper body and the overwhelming conclusion was that they had died from crushing bites from a dog.
"Going from the information about where they were found it is almost certain a dog, or two, were roaming at night hunting penguins, giving a killing chomp and running on to the next," he said.
The dead birds had been found about one kilometre from a proposed public viewing area which the trust had spent the past year working on.
Trust chairwoman Kerry-Jayne Wilson said their deaths were a major setback for those involved in the programme.
"Almost all penguins killed at this time of year will be breeding birds preparing for the breeding season that is about to begin.
"It is these birds that have the greatest affect on the population's ability to grow.
"This could set our Cape Foulwind project back years," she said.
The deaths are being investigated by the Department of Conservation and local dog control officers.