Kaka were once abundant but, by 1930, were localised to a few areas.Threatened kaka have been killed by rat poison at the Pukaha Mt Bruce sanctuary.
Six kaka were found dead in the forest, where the native parrots had been seen "tampering" with bait stations, the Conservation Department said.
DOC's threats unit manager Jeff Flavell said the dead birds were found late last year, and toxicology results just back showed three had ingested the rat poison diphacinone.
There were about 100 kaka living in the 942-hectare forest, and some had been seen getting into the ground-based plastic distribution systems containing the cereal-pellets, which were laced with poison.
Staff had begun adding metal plates to the more than 1000 bait stations within the reserve to stop the birds opening the plastic containers, he said. No more birds had died since.
He said DOC had alerted regional councils and staff across New Zealand to what had happened, so they were aware of the possible dangers to kaka. They had also asked staff to check for signs of similar behaviour at other sites.
"Kaka don't normally break into bait stations but they are intelligent, powerful birds and we need to factor this incident into our management techniques for kaka in the future."
Last year, 14 kiwi were killed by two ferrets at the reserve, forcing the centre to modify its protection programme and change its traps. Without poison, rats and other predators would effectively wipe out native bird populations, which were now growing in protected reserves, Mr Flavell said.
Kaka were once abundant but, by 1930, were localised to a few areas. They are still reasonably common in Whirinaki and Pureora forest parks, but even there their numbers are thought to be declining. They can be found in good numbers on some offshore islands, including Kapiti.