RSPCA NSW chief veterinarian Liz Arnott said the vaccinations helped safeguard cats from the flu, as well as panleukopenia virus - which was common in shelters.

"These illnesses can spread rapidly in a shelter, leading to large-scale mortality, which is a scenario we are committed to avoiding," she said.

The decision came in the lead-up to the feline mating season, during which the cat intake across RSPCA shelters in NSW could increase to 500 a week - eight times the average figure.

All sites will enforce the rule, except for shelters in the Blue Mountains and Illawarra shelters due to their council pound obligations.

The remaining vaccination stock will be used to prioritise cats already in RSPCA care across NSW.

In response to the vaccine shortage, RSPCA Victoria has paused the delivery of feline inoculations for privately owned cats in community outreach programs in order to prioritise its shelters.

"Thankfully, the shortage has not impacted our ability to admit and care for cats at this time, however we are closely monitoring the situation and will make changes as necessary," a spokeswoman said.

RSPCA Queensland is also continuing to accept surrendered or sick animals at its shelters, although the organisation is also experiencing a shortage of vaccines.

"We are monitoring the situation and hope the shortage does not impact our future intake ability," spokeswoman Emma Lagoon said.

Dr Arnott urged people to educate themselves on responsible cat ownership practices coming into kitten season.

A pair of undesexed cats could lead to thousands of kittens being born in just a few years, she said.

"(That is why) desexing is vitally important to prevent unwanted pregnancies," she said.

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