Ellison said his ex-girlfriend had brought violence into his home and had told him she had suffered abuse at the hands of her mother throughout her childhood.
He believed no one connected to the victim was his friend on Facebook and therefore she would not see the post.
But only hours after the online status update went up, the woman's relatives had alerted her to it.
The post began by addressing "those in the Dunedin area" before naming the victim.
"She can only be described as a lethal injection of toxicity who walks between raindrops looking down her nose at everyone she comes in contact with," Ellison wrote.
"This person is, according to her own children's stories, the worst child abuser I have ever heard of."
The defendant said she had engaged in the "systematic, manipulating abuse of all her children from a very early age" and that "she adds nothing of value to the world, having never worked a day in her life".
The victim told the court it affected her "immensely" and had a detrimental impact on her ailing health.
She confirmed she had no convictions for violence, or anything else, and her daughter claimed she had not made any disclosures to Ellison about alleged child abuse.
"I thought it was personally absolutely disgusting," the victim said.
Ellison, who now described himself as "a full-time musician", said he had written the post to try to get his former partner the help she needed. He did not intend it to cause harm to the victim.
"You must have wanted people who read the post to think poorly of [her]. Why else would you post such a vitriolic rant?" police prosecutor Tim Hambleton asked.
Judge Flatley said there was no reference in the lengthy Facebook message to getting assistance for anyone, as the defendant had suggested.
The final line only spoke of karma, he said.
"I take from that Mr Ellison would like to see something happen to the victim that might be detrimental to her well-being," the judge said.
Facebook posts of that sort spread "like wildfire", the judge said, and the harm to the victim was clear.
Judge Flatley sentenced Ellison to 75 hours' community work.
Counsel Noel Rayner said his client had links to rehabilitation residence Moana House and would look into serving his sentence there.
Where to get help:
If it is an emergency and you or someone you know is at risk, call 111.
• Women's Refuge: 0800 733 843
• Victim Support: 0800 842 846
• Lifeline: (09) 522 2999
• Family Violence Info Line: 0800 456 450