A ferret has killed half of the kiwi birds reintroduced to Hunua Ranges earlier this year, in a setback to conservation efforts.
Three of the six kiwis were lost as prey, Auckland Council said on Tuesday.
"This is sad news and a blow to the partners, staff and volunteers who have worked hard to release kiwi into the Hunuas," Mayor Phil Goff says.
"It shows that one ferret is all it takes to undo the enormous effort that has been made to establish a new home for kiwi in south Auckland.
"Establishing a thriving kiwi population in a new location is a complicated process and we need even stronger measures for eliminating pests in the Auckland region."
Regional parks manager Rachel Kelleher says the council hadn't known a ferret was active in the area before releasing the birds. However, once it realised the kiwis were dead, the council took action.
"As soon as we knew we had a ferret in the area we immediately amended our trapping programme to target this species and successfully caught a large ferret soon after in the area where the kiwi were preyed on," she says.
"These types of situations are part of the challenge of managing kiwi in the wild - if it was easy, they would be abundant.
"While this is an unfortunate event, the loss of birds during reintroduction was not unusual or unexpected, but rather part of the challenge of managing species outside of predator-free islands."
The kiwis died out in the ranges over 40 years ago because of pests and predators.
"Basically they've been killed by predators," says kiwi handler Jonathan Miles. "The stoats kill the kiwi chicks, and other larger predators like ferrets will kill the adults."
Ms Kelleher says that while stoats are the main predator of kiwis, the trapping programme in the Hunua Ranges has been enhanced to specifically target ferrets.
A conservation dog will also be used in coming weeks to ensure that no ferrets remain in the area.