Exactly what happened to the dog is unclear due to the different stories Jackson gave about what happened, the judge said.
According to the first account, Jackson was at home alone with his partner's dog, Millie, and decided to put her in the bathroom while he went outside for a smoke, as she had a history of trying to escape the house.
When he went to check on the dog, she was hiding behind the toilet and had knocked over the toilet brush, spilling liquid on herself and the floor, the judge said.
Having cleaned up the mess, Jackson said he noticed Millie inching towards the open door, so kicked it shut to avoid her escaping.
The door connected with her head, and Jackson, who said he also believed the dog might have dislocated her hind leg and didn't know where the nearest vet was, decided to end her suffering by wrapping her in a towel and punching her in the head twice.
But four days later Jackson gave another version of events, saying he was annoyed at the dog so threw her into the bathroom.
He "didn't see how she landed but attributed her rib and hip injuries to that", Judge Tompkins said.
He then reverted to the earlier version of finding the dog hiding behind the toilet, saying he wanted to punish her so wrapped her in the towel and hit her head against the floor twice.
Jackson said he "probably did it a bit hard" and he "underestimated his strength and how small she was", the judge said.
Jackson told investigators "I did not intend to kill her, just teach her not to bite."
A necropsy was performed after the incident and Millie was found to have suffered injuries consistent with "severe ante-mortem trauma" and compatible with compression injuries and significant blunt force trauma.
She would have died soon after, but not immediately, and would have been in severe pain in between the attack and her death.
Jackson's lawyer Letitia Smith said his actions were "impulsive and reckless" and that "it was an accident".
"In his words he 'bonked it on the head a couple of times'. The dog died as a result of that," she said.
"He had a lapse of judgment, he acted impulsively, and that's all there is to it."
She said Jackson, who has traits of antisocial personality disorder, was "very apologetic" for what he had done.
He wanted to own his own business and a conviction would create a barrier to him achieving his goals, she said.
SPCA prosecutor Anna Barham said the gravity of the offending was "very high".
"It was an act of violence against a very small dog and the dog died as a result," she said.
She opposed the application for discharge without conviction, saying the consequences of the conviction would be minimal and "somewhat speculative".
Judge Tompkins instead sentenced Jackson to six months of community detention, disqualified him from owning an animal for five years, and ordered he pay reparation of $273 for vet bills.
After the sentencing, SPCA chief executive Andrea Midgen said the way in which Millie was killed was sickening.
"Millie was beaten to death in what would have been an incredibly painful and frightening ordeal," she said.
"It's both shocking and upsetting that this family pet spent her last moments being treated in such a horrific and brutal manner that would have caused her considerable pain, suffering and distress.
"There is no excuse for any aggression or violence towards an innocent, defenceless animal, and this kind of behaviour will not be tolerated."
