A traumatised Helensville man whose dogs killed a cat and a goat in "a series of unfortunate events" has been fined and ordered to destroy the animals, even though one dog has been rehomed and the other stolen.

The man, whose name was suppressed by Judge Maria Pecotic to protect him and his wife from further distress, was sentenced at the Waitākere District Court on Friday.

The couple lived on a Helensville lifestyle block with their two dogs, a Belgian Shepherd called Foxy and a brindle-coloured Mastiff named Molly.

One afternoon last June when the man was not at home, the dogs escaped and attacked a cat at a nearby property.

A neighbour witnessed each dog with one end of the cat in their mouths, pulling hard at the animal.

The neighbour threw a boot at Molly and managed to chase both dogs out of the property. The cat died from his injuries shortly after.

The dogs then went on to another property where they attacked a goat.

The goat owner, who had greeted and patted the dogs shortly before, heard an "awful noise" from the goat and saw Foxy biting the goat's throat, while Molly had its stomach.

The owner yelled and the dogs ran off, but the goat had already suffered significant injuries to the face and its intestines were out. A farmer euthanised the goat by shooting it.

A Mastiff Cross dog like this one attacked two pets within minutes and fatally wounded them. Photo / Warren Buckland

The incident was a deeply "regrettable, terrible moment" for the dogs' owner, an experienced dog trainer who had worked with the SPCA, according to his lawyer, Elizabeth Greeson.

When he came home and found out what happened, he went to apologise immediately and take his dogs away, she said.

Greeson said the incident had caused the man significant emotional and mental trauma, and his wife has been depressed and had to leave her government job as a result of the attacks.

Judge Pecotic said the attacks were serious but appeared to be a one-off incident, and the two dogs did not show aggressive tendencies otherwise.

The complainant who owned the cat had described the attacks as a "most unfortunate series of events" that no one could have foreseen, Pecotic said, and both cat and goat owners did not want to see the dogs put down.

She said there were no personal aggravating factors from the dog owner, who was of good character and was in court for the first time.

He had had the Foxy since the dog was a puppy, whereas Molly was a rescue dog that he had adopted a short time before the attacks.

Molly has since been returned to the rescue centre and rehomed, but the Foxy has gone missing - likely stolen, the court heard.

Judge Pecotic ordered the destruction of Molly and Foxy, saying she was "bound by the law".

The mandatory destruction of pets is "always emotional, involving not only victims and their pets but also you, the defendant, who stands before me today," she said, addressing the bespectacled man.

"Unfortunately ... exceptional circumstances are needed to stop the destruction of the animals. This simply doesn't exist," she said.

"I make those orders knowing full well that Molly has been rehomed and Foxy is lost. But this is out of my hands. A law change is needed before I can have the discretion [to spare the dogs]."

The man was fined $2000, with a 50 per cent credit for his guilty plea, remorse, good character and positive actions after the incident, including hosting a restorative justice meeting with the owner of the cat at his home and offering a reparation of $100.