Nine sheep including three pregnant ewes were found dead with their "throats ripped and faces torn".
One sheep with facial bites survived the attack.
Harding said among the sheep was their beloved pet, who had been with them for nearly five years.
"It was upsetting to see our pet Ram Percy with his leg chewed off. He would have died a painful death before being eaten alive," Harding said.
"It was a difficult situation because on a Sunday you don't get the dog ranger working, it was First Security who came to pick the dogs up. I did call for help from the two neighbouring farmers, but they were all unavailable."
After their devastating loss, Harding said she wanted to share her story to raise awareness around owners who let their dogs roam.
"People need to take responsibility for their dogs and make sure their properties are fenced securely or their dogs are kept in kennels."
The Hardings have owned their 65ha farm for the past four years and have farmed both sheep and beef.
Harding said the "butchering" of their sheep was not only a huge personal loss, but also a financial one.
"I'm not quite sure how much the sheep were all worth, but my husband believes we've lost a few thousand dollars, especially seeing as three of them were pregnant."
A spokesperson for the Tararua District Council said animal welfare officers were unable to comment on the matter and said an "investigation was ongoing".
"The incident is under investigation, the council is investigating it at this stage and we will be in touch with the owners in due course."
