Magistrate Louise Taylor described the actions of Michael Patrick Stewart as "excessive in the extreme" on Thursday afternoon, when she sentenced the pensioner to a suspended three-month jail term.

Ms Taylor also fined the 69-year-old Ainslie man $350 over an earlier incident in which he threw a knife at the same dog.

Documents tendered to the ACT Magistrates Court show police were called to Stewart's home in response to an incident on New Year's Day in 2021.

The pensioner, who had just returned from a trip to Bathurst, told officers his son's black cocker spaniel, Louie, had damaged property including a prosthetic leg while he had been away.

He said he had found Louie on his bed and told the animal to leave the room, but it refused to move and bit him on the wrist.

Stewart responded by grabbing a knife from the kitchen and trying again to get the dog out, but Louie bit him again on the arm.

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Ms Taylor said that at this point, while Stewart clearly had other options available to deal with the disobedient dog, the Ainslie man decided to stab the animal four times.

The final blow punctured both of Louie's lungs and pierced the dog's heart, and police located the dead animal beside Stewart's bed with the wooden handle of the 34-centimetre knife protruding from its stomach.

Police notified the RSPCA, which launched an investigation and examined the dog's body.

One of the organisation's veterinarians discovered Louie had suffered injuries on at least two occasions, with lacerations on its left hip unrelated to the fatal stabbing attack.

During a subsequent interview with inspectors, Stewart admitted he had thrown his pocket knife at Louie and struck the dog some time in December 2020.

He insisted he had killed the cocker spaniel in self-defence during the more recent incident, stating "he did not believe he had any other options but to attack and kill the dog".

Stewart did not mount a self-defence argument once his case reached court, however, pleading guilty last November to charges of aggravated animal cruelty causing death and hitting, kicking or throwing something at an animal.

The court heard on Thursday that Stewart's adult son had entrusted him with looking after Louie, and Ms Taylor described his conduct in relation to the cocker spaniel as "largely inexplicable".

She said he had "exercised terrible judgment" and resorted to a "wholly inappropriate" way of dealing with a non-compliant dog.

"The act in killing the dog was ... deliberate, in my view, and brutal," the magistrate said.

Ms Taylor added that the 69-year-old also had his own dog, a kelpie named Pippa, with which he had experienced great joy.

She noted Pippa had been seized from Stewart, who was now "very lonely", but said owning an animal was an honour and a privilege the offender had lost the right to have.

In addition to the suspended jail sentence and the fine, Ms Taylor banned Stewart from purchasing, acquiring, keeping, caring for or controlling an animal for five years.

The order was backdated to begin on the day Louie died.

Stewart was also ordered to be of good behaviour for a period of 10 months.

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