An Invercargill woman with more than 20 cats on her property has been convicted of failing to reduce the number of animals and creating a nuisance - and given 21 days to reduce the number of cats to three.

Averil Gardiner was also fined $400 in a decision delivered by Judge David Saunders in the Invercargill District Court at the conclusion of the two day trial. An order for costs was also imposed.

Judge Saunders said Thurso St neighbours of Gardiner had given ample, clear evidence of the ongoing nuisance that "well over 20 cats" had caused and that the Invercargill City Council request for her to reduce cat numbers to the maximum number of three was a reasonable request.

"The time has come now for reality to sink in," he told Gardiner.

Earlier, the court heard from Thurso St neighbours who had complained to Gardiner and the city council about the nuisance caused. Neighbours estimated between 20 to 30 cats at any one time have been present at Gardiner's property since she moved there three-and-a-half years ago.

Neighbour Rachael Webber told the court she resorted to spreading pepper, cayenne pepper and cat repellent to keep cats off her property, but it did not work.

"I'm just so frustrated because we can't enjoy our property."

Webber said Gardiner's house became a spectacle after media coverage of the issue, with people driving past for a look, some stopping to take photos.

Another neighbour, Eric Galbraith said Gardiner's cats fought with other cats, including on his family's roof at night. Galbraith said their 17-year-old cat Snoopy constantly got into fights after the influx of cats. They had to put him down eventually because Snoopy could not defend himself and sustained a lot of injuries, he said.

Gardiner would sometimes let the cats outside in batches, Galbraith said.

"She would let them out in colours. You would see a whole lot of ginger ones come out and then a whole lot of tortoise-shell ones."

Former neighbour Katrina Robertson, who lived next door to Gardiner until December, said her cat Mr Bojangles' behaviour changed after Gardiner moved in. Mr Bojangles stayed inside and a vet told her the cat was stressed. Since moving away Mr Bojangles had been fine, she said.

Defence counsel Simon Claver questioned witnesses about how they could be sure it was Gardiner's cats on their properties. Each responded that when cats were chased away they were observed running back into Gardiner's property.

After the case, Gardiner said she had no comment to make.

Invercargill City Council environmental health manager John Youngson said he was pleased for the neighbours but said it would be a devastating result for Gardiner. The decision created a clear path for resolving the issues and sent a clear message that the bylaw would be enforced, he said.

Robertson said neighbours would believe it when they saw it as the issues had been ongoing for nearly four years.