The Deerstalkers' Association says it is appalled that four protected takahē have been shot on an organised cull targeting pūkeko.
National president Bill O'Leary said he accepted that a Department of Conservation (DoC) investigation would determine whether the birds were killed by a member or members of the association.
He said he shared with DoC a concern that the deaths would affect efforts to save an endangered species, and he apologised to the department and the country at large.
The takahē were shot on Motutapu Island in the Hauraki Gulf yesterday.
Takahē are roughly twice the size of pūkeko, and flightless.
Listen to the sound of a takahē
As recently as 1948, they were thought to be extinct, while at the beginning of 2013, their population stood at 263. The population currently stands at about 300 birds.
Takahē chicks were successfully hatched on Motutapu in 2013, which DoC's takahē recovery group manager Deidre Vercoe Scott said, at the time, was a "huge event in the work to save takahē".
"Motutapu is a key site for the Takahē Recovery Programme. It's a pest-free island with a suitable habitat that's large enough to accommodate up to 20 breeding pair of takahē."
DoC hoped the island would have the largest population of takahē outside the Murchison Mountains in Fiordland.