"Often there's a really big surplus of kittens, and Christmas is a really good time to think 'oh, lets just give someone a kitten'," Midgen said.
"Unfortunately a kitten is quite a big commitment - they can live for 18 years, so people need to be really responsible and think; 'can I commit to having an animal as part of our family for all that time?'."
She said the cat population could mushroom fast, and desexing pet cats was the best way to ensure there were enough good homes for kittens.
"It's amazingly quick; from one mother they can produce another 12 kittens over a season, and then if you multiply that out, then by the end of four years it's another 2100 kittens that could be around and not have a good home."
NZ Veterinary Association small animal manager Lorelle Barrett said estimates showed there were about 1.1 million domestic cats in New Zealand, and almost 200,000 stray cats - cats that are socialised to humans but don't have a home.
The number of feral cats is less clear, but in 2016 the National Cat Management Strategy Group referred to a study estimating it could be as high as 2.5m.
"De-sexing of domestic cats helps control the cat population by reducing the number of potentially unwanted stray or feral kittens that might be born, and go on to predate on wildlife," Barrett said.
"It is also in the interests of cat health - the health and welfare of feral and stray cats can be significantly compromised by diseases and health conditions that domestic cats often receive preventative health care for."
Domestic cats were more likely to wander further and fight more often if they are not desexed, she said.
Low-cost Snip 'n' Chip desexing clinics have been run by the SPCA for more than four years, demand is high, and they are continuing to expand.
Four more opened in December - Ruapehu District, Southland, Whakatāne and Kawerau.
"We desex them and microchip them, so they can always find their home. We've done around 30,000 this year, and the impact is quite a significant reduction in the number of kittens," Midgen said.
"Also there's lots of other rescue groups around the country which are doing the same thing, so it does have a really huge benefit. But the problem is that you miss one cat, and you've got back to huge numbers very quickly again."
Extra Snip'n'Chip clinics have been targetted at areas where very high numbers of kittens have been brought in to SPCA shelters - including Manurewa, Papatoetoe, Mangere, Te Atatu and Titirangi.
"We've targeted those suburbs over the last four or five years, and what we've seen is a huge reduction in the numbers of incoming kittens because of Snip'n'Chip - probably as much as 50 percent reduction," Midgen said.
"We would love people to donate to us to enable us to desex more, it costs somewhere between $150 and $200 to desex every kitten or cat we get across our door, but we have a really good process place, so the more money we get, the more we can do."
- RNZ
