Pets n Vets co-director Dr Ngaire Mace said it was important to castrate cats to prevent them from breeding, especially at this time of year.
Entire male cats tended to fight each other over territory and were prone to spreading diseases, including feline immunodeficiency virus, or "cat Aids".
"We decided there were a lot of people in town who may not have the financial means to have their cats castrated and we tend to end up with a lot of problems," Dr Mace said.
"It's a surgery we take extremely seriously and we want to raise awareness that cats should be castrated.
We want to give something back to the community after all their support and it's great for staff morale and teamwork."
The free cat castration day was successfully trialled by Pets n Vets at its Christchurch clinic six weeks ago and at its Ashburton clinic two weeks ago.
Castration usually costs $65.
Dr Mace said at least two stray cats were brought into the Queenstown clinic by the public for treatment and shelter every week and she believed many more were roaming around the Wakatipu.
Queenstown's transient population often meant owners left their moggies behind or released them on to the streets.
Two veterinarians and others on stand-by will offer advice on cat welfare, examine the animals and offer vaccinations against cat flu and enteritis, plus flea and worm treatments at reduced cost.
Between 50 to 70 cats were expected tomorrow.
The animals would be anaesthetised, surgery performed, then they would ready for collection once they had woken up in the afternoon.
