If more people don't step up to foster animals, the SPCA will reach crisis point as centres around the country overflow.
Auckland SPCA chief executive Andrea Midgen said the organisation is in desperate need of volunteers to foster hundreds of animals.
"We can't take in any more animals," she said.
The Auckland centre is completely full, with no room in the hospital and isolation areas for incoming animals. The centre is sending out 180 animals on average to be fostered each month.
Meanwhile, in Wellington, there are 136 animals being fostered, and 80 in the Canterbury centre.
Foster parents don't have to take on any costs, as the SPCA will provide everything they need, from medicine and vet costs to food, toys, and bedding.
Fostering is for animals that "need a bit of rest and respite".
They could be sick, recovering from an operation or procedure, need some time in isolation, or need regular care such as bottle-feeding every couple of hours.
Anyone wanting to foster can work with the SPCA around the level of commitment needed to care for a certain animal, and could choose to foster for just a weekend, or for several months.
Normally by this time of the year, the kitten season has calmed down and foster parents can get a breather, but the number of incoming cats has stayed high, Midgen said.
Wellington SPCA staff member and foster parent of 13 years, Kirsty Grant, is looking after three kittens at the moment.
She also owns four cats that are all ex-foster cats "that never quite made it back".
One is 5-year-old Milo, who was born with an umbilical chord wrapped tightly around his leg, cutting off the circulation so badly it needed to be amputated.
The kittens she is caring for at the moment are sick, but can feed themselves, so Grant doesn't have to do much except play with them and take them to vet appointments, which the SPCA pays for.
There were plenty of benefits to fostering cats or dogs, including the ability to trial having a pet in the family home, or the chance to look after animals for a short period of time, rather than commit to having a pet for 10-15 years.
"I would just say give it a go," she said.
"Everybody that we've spoken to and that have stepped up as fosters have really, really enjoyed it, even if you only foster once."
Anyone wanting to foster should contact their local SPCA centre.