Dying toddler’s step father left him home alone to go and have sex.

Mason Lee should be a happy, lively two-year-old. He should be playing with his toys and laughing with his siblings.

He should be adored and cherished, babbling about his beloved Batman, excited about the Christmas ahead.

But instead he died in June, at 21-months-old. A sickening, painful death that could have been prevented if he had been given medical treatment.

The toddler died from faecal peritonitis - the leakage of faecal matter into the abdominal cavity brought on by a perforated abdomen, but his injuries had been there for a substantial period of time.

When paramedics were called in the early hours of the morning in June they allegedly found Mason covered in vomit, wearing a faeces-stained singlet.

Under the singlet his abdomen, arms and legs were battered, bruised and scarred.

Blood had pooled around his neck and ears, his eye was covered in a bruised, his skin was glazed, red and puffy.

But it gets worse. Organs were ripped from his abdominal cavity and his arms appeared to be ruptured, and according to The Courier Mail his anus had been torn open.

A medical examiner found there had been separation of connective tissue on his skull – most likely from having his hair pulled.

The youngest of five children Mason Lee spent the last weeks of his life living with his step-father William Andrew O’Sullivan. Mason’s mother lived several streets away with her four other children.

He had been sick for days but his mother, Ann-Maree Lee,  step-father William Andrew O’Sullivan, and 17-year-old Ryan Hodson who lived with Sullivan had been allegedly arguing over whose responsibility it was to take him to the doctor.

Just days after his painful death, Moreton Police Detective Inspector Paul Schmidt said police believed Mason Lee died from a blood infection caused after his abdomen swelled when his small intestine ruptured.

"Police will allege that the child died as a result of peritonitis, bought about by a ruptured duodenum," Inspector Schmidt told reporters.

"The injuries to the child would have been obvious, would have been deteriorating and would have been significant over a period of time," the senior detective said.

They arrested his mother, Ann-Maree Lee, her boyfriend William Andrew O’Sullivan, and 17-year-old Ryan Hodson who were all charged with Mason’s manslaughter, with police alleging they did nothing to save him as he slowly died from internal injuries.

It has been revealed in court that the three had exchanged text messages arguing over who should help the toddler with a photograph taken of Mason covered in vomit and dying by O’Sullivan sent back and forth between the three.

None of them volunteered to seek medical care.

Yesterday further details emerged on the night the 21-month-old died reports The Courier Mail. 

The court heard that while the vomiting and sick 21-month-old lay in excruciating pain he was left alone for an hour with only a five-year-old boy while his stepfather, William O’Sullivan, went out for sex.

Prosecutor Sergeant Liesl Stephenson said O’Sullivan left the children alone and drove to where Mason’s mother was living so they could have sex.

She told the court: “CCTV footage shows the defendant leave the children unsupervised after 10pm on the night of Mason’s death. He returned with his friend about 11.30pm.”

An hour later he was dead.

She told the court that O’Sullivan “on one occasion” rubbed Mason’s face in his own vomit.

Sgt Stephenson said O’Sullivan also took a photograph of Mason with a puppy lying on his neck and vomit beside him, and texted it to a number of people.

“He takes the time to photograph a 21-month-old Mason dying in vomit,” she said.

“That’s the extent of care that this defendant has shown to a child who is clearly dying.”

Sullivan was in court seeking bail, he was refused, with Magistrate James Blanch saying he was safer in prison given the "community outrage" over Mason’s death.