One of the country's top wildlife vets has criticised the SPCA for neutering stray cats and sending them back into the wild, saying it goes against protecting native species.
Some regional SPCA branches have a trap, neuter and return programme for stray cats.
There were no records kept on the number of stray cats in New Zealand, but it was estimated that there were 20,000 colonies in Auckland alone.
Brett Gartrell, the co-director of New Zealand's only dedicated wildlife hospital, Wildbase at Massey University, told Nine to Noon releasing the cats went against conservation efforts.
"If they're the society for the prevention of cruelty to all animals and not just domestic pets, then they need to consider the welfare of wildlife," Mr Gartrell said.
"What the TNR programme is saying (is) the welfare of cats is more important than the welfare of the native wildlife."
But the SPCA said only 2 percent of the 15,000 felines handled last year were neutered and released to the wild, and it was not a pest control organisation.
The SPCA defended its strategy of neutering stray cats and sending them back into the wild.
Chief executive Ric Odom said it was one of the only groups trying to help.
"We're not adding to the problem, the cats are already there," Mr Odom said. "Saying that the SPCA is contributing to the deaths of native birds is just wrong. The cats are there already.
"What we're trying to do is to utilise a number of options - and try and control it."
Mr Odum said only a tiny number of cats were neutered and returned each year.