An investigation by the Ministry for Primary Industries (MPI) has found evidence of horses being illegally castrated on farms in Northern Hawke's Bay and the East Coast.
An MPI district compliance manager, Mike Green, says unqualified people rather than veterinarians are operating not only on their own horses but as part of a business.
He says it appears the practice is ingrained in the culture of the farming and rural community.
Mr Green says there are no excuses for owners of horses not employing the services of a qualified veterinarian.
"People who we have identified as undertaking this practice and the people who have engaged the services of these people have all been spoken to and at this stage we have issued up to nine warning letters to people to advise them it is illegal."
He says they've been warned that if the illegal operations continue to be carried out they might be prosecuted.
Under the Animal Welfare Act people carrying out illegal castration can be liable for a fine of up to $50,000 dollars or a prison term of up to 12 months.
The Equine Veterinary Association says it's inhumane for anyone but a qualified veterinarian to castrate horses.
Association president Barry Drayton says it's unacceptable to perform surgery on a horse without using the right drugs to control pain.
