Afterwards, he was put into the back of the friend's car where the woman's 3-year-old son was strapped into his car seat.
Nig bit the boy on his face, ear, neck and shoulders, causing puncture wounds, the court heard.
"There is no evidence of anything done by the child that may have provoked an already irritated dog. On the facts as I have them it was an unprovoked and sudden attack,'' the judge said.
Defence counsel Jo Turner argued there were "exceptional circumstances'' which should allow the pet's life to be spared.
Nig went missing for a day in November and returned with an abrasion to his left hind leg.
Rewi took him to the vet where she was told the limb might be fractured, but she could not afford the cost of treatment.
Turner told the court Nig's running at the park could have exacerbated the injury and made him more irritable.
But Judge Walker was unconvinced.
"While it is a possibility that Nig was uncomfortable or in pain it is no more than a possibility. There is no evidence of the dog exhibiting any discomfort prior to being put in the car,'' he said.
"There is no basis for me to be satisfied that the circumstances were exceptional.''
Veterinary records produced for the court in support of the defence eventually worked against Rewi's case.
The judge noted on the files, staff had written: "Watch this dog as he is growly'', and they had also recorded that Nig was muzzled for a subsequent exam.
In September last year, Rewi had been issued a final notice for her dog wandering and ordered to fence her property. That month Nig was involved in an incident with a neighbour mowing the lawn, the court heard.
Since the attack, the animal had been in the council pound, Rewi clocking up a $2500 debt.
Since the defendant was of limited financial means, Judge Walker refused to compound her pain by fining her.
"I take the view that the consequences of Ms Rewi losing her dog is a very substantial consequence in itself.''
