"They have to catch what they eat. That's not their fault, it's just their hunting behaviour."
The problem was that while groups worked to trap possums, stoats, mice, and rats, "the apex predator" was still roaming around, and it wasn't always easy to tell what was a pet and what was feral.
There was no mandate to microchip cats, which caused problems when attempting to return pets to owners, and by-laws would make it easier, he said.
"Some people will think, 'Oh, you're wanting to get rid of cats, or keep them stuck in a room', when that's not the case at all."
It was about education and awareness, he said.
"I'm hoping council will start having a think, like other councils are, about some by-laws ... which would be specific around feral cats."
Dumped pets were driving the population numbers, he said.
Whakarewarewa Pest Free representative Anthony Garea agreed there was a need for a large-scale and consistent programme to control feral cat numbers, but was less sure about microchipping mandates.
Garea said keeping them from breeding would be most helpful, and getting people to understand feral cats were not a "little moggie".
Other areas of the region also experienced cat-dumping problems.
Te Whakakaha Trust looks after 400 hectares of forest at the Otawa Scenic Reserve near Te Puke.
Trust chairman Jim Dowman said there was a consistent problem of people dumping unwanted pets.
"People come up the No3 Rd for 12km, then there's a gravel road and bush, and a river, and they think, 'Oh, this is an ideal place to chuck our cats out.'"
They soon became wild animals, he said.
The trust used a trap to catch them, releasing what it believed to be domestic cats and disposing of the ferals.
It was not easy to know how many feral cats were in the region. Bay of Plenty Regional Council biosecurity officer Dale Williams said estimating their numbers was not able to be done reliably.
He said the council may provide advice on how to manage or control advisory species if required, but did not enforce control.
It did not carry out specific feral cat control but did support pest-control programmes that may include cat control such as the Ōhope kiwi project.
"The issue of feral cats is ongoing and will continue as long as there are un-neutered cats and kittens that are abandoned."
The impact feral cats had was complex, he said, and difficult to quantify.
"In the wild, cats will kill and eat a significant number of rats and mice which also prey on native biodiversity. They are also implicated in the spread of the disease toxoplasmosis."
When asked what policy or restrictions were in place or suggested in the past, he said one of the biggest problems was establishing ownership of trapped cats.
"Some city councils such as Wellington City Council have introduced by-laws requiring cat owners to neuter and microchip their pets. Undertaking cat control is significantly easier because cats with microchips can be returned to their owners, while unowned cats can be rehomed or euthanised."
He said the council supported responsible cat ownership which included microchipping, desexing, and keeping cats contained at night.
SPCA area manager Sue Kinsella said growing stray and feral populations were a problem in New Zealand.
She said it was illegal to dump or abandon an animal, but it was hard to determine what was lost or abandoned without evidence.
Any cats that came to them were checked for microchips and listed on lostpet.co.nz.
They receive vet treatment if needed, and all cats and kittens adopted from SPCA are desexed, microchipped, and vaccinated before adoption.
Last year, SPCA desexed more than 28,000 animals.
"While we've made huge progress in this space, stray cat populations are still a complex issue that we're determined to continue to work to address."
