So far more than 200 dogs and a similar number of cats have had free operations in an attempt to reduce the number of unwanted puppies and kittens in the region.
Central Otago SPCA co-ordinator Shirlene Steel said veterinary clinics had received a ''massive'' response to a story about the programme published in the Otago Daily Times in July.
The free operations were designed to give lower-income households the chance to desex animals, although anyone could have an animal desexed during the campaign.
Waiting lists for desexing at some clinics were still ''quite long'', Ms Steel said.
She hoped the number of animals operated on would represent more than 50% of eligible animals in Central Otago.
The operations would mean the SPCA, which has a non-euthanasia policy, would save money in the long run.
''The fact that there are so many animals that have been de-sexed is a success ... That's just great.''
While the desexing was free, animal owners had been encouraged to make a donation in lieu of payment.
''Lots'' of people had made ''better than expected'' donations, and those who could afford it were generous, giving up to half the cost of the operation, Ms Steel said.
Income from donations had been used to pay for microchipping and registration of any animals being desexed which needed it, she said.
Ms Steel had expected more dogs to be signed up for the operations than cats, but the numbers had been ''fairly even''.
It was expected the programme would be completed in about a month, and its success would then be assessed.
The SPCA would ''definitely'' consider providing free desexing again, if it could obtain funding, Ms Steel said.
She thanked the Pelorus Trust for funding the programme, and veterinary clinics across Central Otago which had worked to deliver the operations alongside their daily work.
