Warning: Some readers may find the contents of the letter distressing.

Tenants of a Brisbane apartment complex have been left disturbed after a threatening letter was sent to them by an anonymous neighbour over the alleged mistreatment of a dog in their building.

The letter, which was slipped into the mailbox of all 154 units in the Alto Apartment in Toowong on Friday, called out the dog's owner for causing it a "painful life of abuse" because of the dog's incessant barking.

But the "insane" letter, which the neighbour has made violent threats against the dog and its owner, has left tenants distressed and on edge.

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The bizarre letter begins with 25-lines, filled with the word — 'Bark' — before the author translates the passage.

"This roughly translates to 1) Help I am in prison! 2) My owner is a f***wit 3) I want to jump off this building and die," the neighbour wrote.

According to the author, the poor pup has been barking non-stop for weeks, but it only starts after it's owner leaves for work each day.

"You must not (have) been loved enough as a child or f***ed enough as an adult," they wrote.

"You are truly a sick twisted individual and should not be allowed to own pets.

"You are everything that's wrong in this world."

The letter featured long ramblings from a stranger, directed to everyone in the apartment complex.
The letter featured long ramblings from a stranger, directed to everyone in the apartment complex.

The letter continues, revealing that the author has already contacted the Brisbane City Council about the matter, and won't hesitate to call the police if action isn't taken.

"I would love to lock you on a balcony for hours on end, with or without food and water for weeks on end and see how you cope," they said.

The author then made threats against the poor pup's owner, who they labelled a "rich wanker" and a "f***wit".

"I will hire lawyers or barristers or QC's to stop this evil act from occurring," they said.

"I will contact channel 7 … 9 … 10 … human rights … animal rights … PETA … I will bring you down."

At this point in the note, the author's obvious rage at their neighbour spills over, and the letter descends into some pretty graphic threats.

They said they wanted to scale the 20-storey building "and throw the dog off" so that other tenants could see that the animal had been saved.

But a source told news.com.au that the recipients of the letter hadn't heard a dog barking and thought "it was insane".

Tenants have been told that Alto Apartment's management is investigating the claims in an attempt to figure out who composed the offensive note.

News.com.au has contacted Alto Apartments for comment.