Federated Farmers meat and fibre chair Miles Anderson issued a press release saying he was "mystified and frustrated" by the attack.
"The implication that shearing sheep is cruel or mistreatment is mystifying to most Kiwis, let alone farmers," he said.
"Sheep naturally grow wool and if we didn't shear them it would cause great animal welfare issues, such as fly strike or discomfort having to carry a 5kg plus fleece around in the heat of summer.
"Shearing is like getting a haircut, simple as that."
A British professor of animal welfare who is in New Zealand on a year's secondment with SAFE (Save Animals From Exploitation), Dr Andrew Knight, said he saw sheep being shorn in Queenstown recently and sent out into the cold without their wool.
"They are manhandled and put into positions that are awkward for them. This is stressful for them," he said.
"Then the wool is removed quite quickly, and usually skilfully of course, there might be a few nicks in the skin, and they are released and go back into the flock,
"They don't show any obvious signs but they experience the shock of cold and stress. That is not pleasant. We tend to forget all that because these animals are so stoic, they quietly take it all."
He said sheep had evolved not to show obvious signs of distress because that would have made them more vulnerable to predators, but research had found that stress hormones such as cortisone shot up in the shearing process.
Knight, who is originally from Australia, said his biggest concern for sheep was mulesing, a common practice in Australia which involves cutting strips of skin off the animal's hind quarters to stop faeces collecting and attracting flies.
His second-biggest concern was docking lambs' tails without using painkillers.
"Painkillers are sometimes used by the best farmers. They should be used by all farmers," he said.
He also criticised dogs which herd sheep by "menacing" them, and lambing in the cold winter months.
"I don't wear wool," he said. "I'm into mountaineering. I like lightweight products that keep you warm when you're wet. Synthetics will do that."
However Anderson said mulesing was illegal in New Zealand and he believed there was no need for painkillers in docking.
"Most people use rings so the tail goes numb, I guess," he said.
He said lambs were born in August and September because ewes "only start cycling in March, and if you count five months from March you're into August".
As for shearing, he said shearers used wider combs which left more wool on the sheep when shearing in colder months.
"Farmers are not going to be shearing animals to have them go out and die, because it's our livelihood," he said.
"Shearing with crossbred animals, which is 26 million out of 27 million sheep in New Zealand, is not an economic prospect and hasn't been for quite some time, so farmers are shearing the animals simply for animal health because the returns from wool don't cover the cost of the shearing at the moment."
Farmers still earn income from sheep meat and many, like Anderson, also have other sources of income such as crops. But he said the low returns from wool accounted for the dramatic decline in NZ sheep numbers from more than 60 million a few decades ago.
