"Certainly the welfare side of things is of concern, pain relief is the first and foremost thing," he said. "The risk of bleeding and complications and infections if it is not done correctly is high."
Horses were given general or local anaesthetic by vets to sedate them. "They are completely out to it, but if people were doing it themselves they would just rope the horse down and just cut them out. I'm certain they would be in a fair bit of pain.
"Even the fact of roping them down, if they are struggling, which I can imagine they would be, there could even be complications from the roping."
He said it ranged from between $350 to $400 to get a horse castrated, and they were often vaccinated at the same time.
Local horse breeder Graeme Hart, from Fernhill Stud, said there was no excuse for farmers not to use professionals. "It doesn't cost a lot to get it done and the vets are very good and have got lovely drugs to use, it just takes out any risk," he said.
SPCA Hastings branch president Sharon Arcus said it was often done as a cost-saving method, at the expense of animal welfare. "When a procedure is not done under vet care and under anaesthetic there is concern about the pain the animal might suffer. There will always be people who cut corners at the expense of animal welfare. As president of the SPCA I am really concerned and do not condone the choices that these people are making."
Those caught doing or commissioning the illegal castrations could faced fines of up to $50,000 or 12 months in jail. Mr Green said warnings were issued to some of those dealt with by MPI, and prosecution was being considered against others.
Report cases of animal cruelty and breaches of the Animal Welfare Act to the Animal Welfare hotline 0800 00 83 33.