South Island drought conditions could be reducing the frog population.
Chief scientist of the international campaign group Amphibian Survival Alliance, Phil Bishop, said this year's weather had been so dry that frogs were unlikely to be breeding in their usual numbers.
The South Island's two introduced species, the southern bell frog and the brown tree frog, relied on damp conditions to breed.
Dr Bishop said frogs were a vital food source for some birds, so if their population fell too far, the ecosystem could get out of balance.
"I think this is just another straw on the camel's back.
"We find that amphibians are declining around the world. So particularly worrying are our native frogs in New Zealand that only live in the cool, mist forest of natural bush. And that's going to become few and far between in the next 50 years."