i was staying on a small property near melbourne a few weeks ago. one morning i looked out the back window and could see 8 cats. i also was walking along a creek, some distance from the house, and finding numerous cats, not even trying to hide themselves.
the problem is there and definitely needs something done about it. suburban trapping would be a great start, with near immediate euthanising, apart from registered cats, where decent fine would be appropriate. $20 dollars means nothing to most people and wouldn't change the situation. maybe $150 fine would be fare, as a first offence penalty.
maybe a license similar to a drivers license would be appropriate. eg, start with 12 points, first offense you loose 2 points, second you loose 4, third you loose 6, and your right to own cats for up to 3 years, etc. this kind of licensing would also work for animal abuse etc.