The Cat Crisis Coalition, whose member organisations include the RSPCA Victoria, the Lort Smith Animal Hospital and the Cat Protection Society of Victoria, congratulated the council on an "enlightened decision".

Director Carole Webb said the coalition had been lobbying for mandatory desexing of cats for 20 years. "We're happy they've taken that on board."

Ms Webb said cats could have three litters a year, and with each litter they had an average of four kittens. "When you start to multiply those figures out, you can see they can increase at an exponential rate."

She said there were not enough homes for an increasing cat population. "By having them desexed, we're stopping those unwanted kittens from being born."

Ms Webb said no homes were found for three out of four kittens taken to shelters. Victorian shelters take 53,000 cats or kittens each year, of which 36,000 are put down.

"We've tried with education, discounted desexing schemes, we've tried a whole variety of incentives to get people to be responsible for their cats. Very sadly, thousands of cats still keep flooding into shelters each year."

While she would like to see mandatory desexing introduced at the next registration period, rather than 2011-2012 as the council has proposed, Ms Webb said the coalition was grateful it was on the council's agenda. "We appreciate the community does need time to adjust, but if we had our way it would be now."

Knox Mayor David Cooper, who has previously held concerns about the young age kittens were desexed, said the plan would mean cats could be desexed up to 15 months old. He said the plan would also give families the opportunity to raise cats and kittens.

The council's decision to introduce mandatory desexing is part of its Domestic Animal Management Plan. The plan outlines a three-year approach to responsible animal management, including minimising the risks of animal attacks and reducing public nuisances caused by animals. It includes trial projects, such as a cat curfew, which will provide the council with data that analyses local feline trends and behaviours.

Cr Cooper said pets were a great way to enrich a life but came with responsibilities. "The council's plan aims to make pet owners more aware of those responsibilities and help them embrace principles of responsible pet ownership."