Greens lord mayoral candidate Ben Pennings said the mobile desexing service would reduce the size of Brisbane's feral cat colonies and save native wildlife.

"Unwanted cats pose a significant threat to Brisbane's wildlife," he said.

"Kittens are dumped, leading to feral cat colonies in Brisbane's suburbs and millions of native animal deaths each week.

"Feral cats mate with domestic cats, compounding the problem.

"Let's give our native species a fighting chance."

Mr Pennings said under the Greens' Stopping Ferals Saving Wildlife commitment, 20,000 cats would be desexed every year.

The plan would cost ratepayers $6.5 million over the first two years, but Mr Pennings said the council would save money over time.

"Unspayed cats can have up to 12 kittens a year," he said.

"Every cat desexed will reduce council costs in feral cat control and maintaining animal shelters.

"Desexed cats are less likely to roam, saving even more council resources."

Under the program, cat owners would also receive $25 worth of cat food and cat litter.

RSPCA Queensland spokesman Michael Beatty said, on the face of it, the Greens' policy appeared sound.

"Anything that would help more cats being desexed would have to be a good thing, because that would certainly, over time, ease the strain not just on Brisbane City Council pounds but our resources as well," he said.

Mr Beatty said he hoped the two parties with a realistic chance of controlling City Hall – the LNP and Labor – would also include animal welfare in their election manifestos.

"We'd certainly like animal welfare in general to be on the agenda for both major parties," he said.

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