Ferrets have killed four brown kiwis at Pukaha Mount Bruce in the past month, bringing the number of kiwi fitted with transmitters at the wildlife centre down to just seven.

General manager Helen Tickner said the centre will be doing a kiwi call count later in the year and is confident there are plenty more kiwi in the reserve including many females without transmitters.

"Without downplaying it, in an unfenced reserve at this time of year, it's not unexpected to have this predation, but it's horrible," she said.

"We've caught one ferret and we're pretty confident that we're going to catch the other culprit that is still in the reserve."

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Mrs Tickner said all recently killed kiwi were "certainly over the 1.2kg mark, so at least 18 months old".

"It's really distressing for everyone here because any kiwi is just so precious, but we're still confident that what we're doing is important for conservation in New Zealand.

"It just highlights to us again that there's no taking your foot off the accelerator. We are always keeping an eye out for predators because this is what predators do, they hunt, they kill."

She said dealing with predation in the reserve sometimes feels like "three steps forward, two steps back" but "at the end of the day we'll win".

"Our predator control is so important to us and it just highlights why we spend so much time and effort on it. We're always learning, and we'll learn from this even more."

The most recent kiwi hatching was on April 14, and another egg in the nursery is set to hatch within 10 days.