Researchers are struggling to stop deaths of native frogs being kept in captivity in case the wild population dies out.
Out of 83 Archey's frogs kept at Auckland Zoo since March 2005, 42 have died.
A week ago the zoo revealed that five of its six blue penguins had died in the last six months. The frog deaths are unrelated.
The frogs have been kept inside to protect them from chytrid fungus, which has wiped out some Australian frog species and has started killing New Zealand frogs.
But staff working for a joint project between the zoo, the Department of Conservation and a frog recovery group have found it difficult to keep the frogs healthy in captivity.
At 37mm long, Archey's frogs are the smallest of New Zealand's four remaining native frogs. They live only in moist, misty areas of the Coromandel and in one site west of Te Kuiti.
DoC classifies them as "nationally critical" - its highest threat category.
No one knows how many are left, although DoC frog ecologist Amanda Haigh said there could be between 5000 and 20,000.
The captive breeding programme was planned after the frog population at a Coromandel monitoring site plunged 88 per cent in the mid-1990s, probably because of the chytrid fungus.
Zoo spokeswoman Jane Healy said the captive frogs could play a critical role in conserving the species. But while the programme got the frogs to lay eggs, only one froglet hatched, and it died.
She said some of the deaths were from a bone disease caused by a lack of nutrition or UV light. The frogs were being treated with UV lamps in their enclosure, which was designed to copy the natural light of the forest.
Meanwhile, the zoo is looking for a new home for its parma wallabies after six died in the past year.
Ms Healy said the wallabies were from Kawau Island.