The SPCA expects to desex 30,000 animals over two years in a campaign to combat the overpopulation of dogs and cats.
A mobile caravan was starting a two-year journey with stops in the East Coast of the North Island, offering free desexing and veterinary services to pet owners.
National Desexing Manager Rebecca Dobson said SPCA centres and animal rescues throughout the country were feeling the pressure from an inundation of animals, but the problem was particularly bad in the East Coast right now.
"By desexing 15,000 animals [each year] we're preventing 100,000 puppies and kittens from being abandoned and neglected, because those animals won't then be able to go on and have unwanted litters if they've been desexed."
New Zealand had a problem with "excessive uncontrolled breeding" of pets, which was creating widespread issues with stray and unwanted animals, the organisation said.
But for some owners, transport or the cost of desexing were barriers.
The first stops for the caravan would be Ruatōria, Te Araroa, Waihau Bay, Te Kaha, Ōpōtiki, Tokomaru Bay, and Tolaga Bay - some of which were a two hour drive from vet services, the SPCA said.
As well as desexing, the staff working from the caravan could offer microchipping, vaccinations, and flea and worm treatments for free or a koha.
SPCA chief executive Andrea Midgen said fewer animals were desexed during the Covid-19 lockdowns, which had contributed to the surge in litters.
"In areas such as the East Coast and Northland, we are often faced with a heart-breaking decision of not being able to accommodate more animals in our shelters."
There was also a vet shortage in New Zealand, and the SPCA was still working on finding vets and vet nurses to help provide the mobile caravan service, with the aim to keep it going year-round and to extend its visits to areas outside East Coast.
As well as the caravan, the SPCA was helping fund a Northland programme where Kerikeri Highway Vets would work with animal rescue groups to desex at least 200 dogs and 300 cats.
And, the SPCA would continue to run its existing Snip 'n' Chip campaign providing free or subsidised desexing and microchipping for some groups.
Midgen said it would take years for the glut of unwanted puppies and kittens to relent, but desexing and responsible pet ownership were the only ways to get there.
The programme was expected to be extended beyond two years, but this first two years was critical, and it would help provide data and targets for future years.
A booking system for the mobile caravan was available on the SPCA website for those interested in having their animals neutered.