The Ministry for Primary Industries (MPI) is investigating after a woman complained she found a number of "dying and dead" calves on a farm in Waikato on Monday.
Jamie-Lee Mohi says she noticed the sick animals on the outskirts of Taupiri while driving towards Hamilton with her mother-in-law.
A little while later when heading home to Te Kauwhata she passed the farm again. This time, she got out of her car to take a closer look and was shocked to find a number of "sick and underweight" calves on the property.
"The first one I saw was dead, the second one I saw was just laying there with its neck out trying to slowly die," Mohi told Newshub.
"It was the most horrendous thing."
After ringing the SPCA - which referred the issue to MPI - Mohi says she went back onto the farm to look for the animals' NAIT tag numbers. At that point, she says, "I actually noticed the problem was far worse than when I first saw it."
She says she saw four dead cows, as well as "several cows slowly dying" and "more cows that are standing but before long won't be".
"It's absolutely horrific," she said, "I don't have any words."
Mohi said while looking for the cows' NAIT tags she encountered a worker on the farm who told her the cows had just been moved there and didn't belong to the farm. She was also told the animals were dying of worms.
Some commenters online, replying to photos and videos Mohi posted on Facebook, said without more evidence it was hard to know exactly what happened and the farmer should be given the "benefit of the doubt".
One person, who appeared to know who the animals belonged to, said the farmer was "aware of the situation" and had pulled some "orphan stragglers he didn't know about out of the bush", and a vet had been notified.
"Definitely not an ideal situation, but the farmer is doing his best for them," the person wrote online.
MPI confirmed it had received an animal welfare complaint and was investigating, but said it couldn't comment any further at this stage.