But after the dog was exhumed from Polaschek's Halswell property, subsequent veterinary and toxicology reports concluded that the dog would have suffered "at least several minutes" of distress and excruciating pain.
The court heard that after the magnitude-7.1 earthquake of September 4, 2010, the dog had become "distressed" and "unpredictable".
Polaschek had resorted to walking the dog at night to keep it away from other people.
He believed he was giving the dog a "humane death" on June 24 - hours after it bit his grandson at his home - as he could not afford to pay a vet to put it down.
Judge Gary Macaskill fined Polaschek $500, which he ordered is to be paid to the SPCA.
He was also ordered to pay $862 in vet and toxicology costs, as well as court costs.
Polaschek was also banned from owning any animal for five years.