Mum fatally injured Tyrell Cobb: judge

A Gold Coast mother who fatally wounded her four-year-old son Tyrell Cobb then went in search of cannabis instead of help deserves a "very substantial term of imprisonment", a judge has found.

The Brisbane Supreme Court had heard during Heidi Strbak's hearing that she was "a good mother when she was stoned" but was prone to aggression towards Tyrell when she was deprived of cannabis.

Tyrell died on May 24, 2009 after two separate blows to his abdomen that caused internal bleeding and the leaking of his stomach contents, known as peritonitis.

Strbak pleaded guilty to manslaughter in November but denied inflicting the fatal blunt force trauma.

Defence barrister Greg McGuire argued the circumstantial case against Strbak's then-partner Matthew Scown was stronger than against Strbak.

But Justice Peter Applegarth on Monday found Strbak was the perpetrator of Tyrell's abuse and did not seek medical help to avoid a child protection investigation into his "constellation of injuries".

"Strbak was callously indifferent to her son's condition," Justice Applegarth found.

"She left the child with Scown and went in search of cannabis, rather than take him to hospital or otherwise seek urgent medical care which would have saved him.

"She deserves to be sentenced to a very substantial term of imprisonment."

Strbak, 34, could be seen rocking back and forth in the dock and shaking her head as Justice Applegarth handed down his findings.

She will be sentenced on December 18.

The boy was seen at preschool on the Friday with no observable injuries other than a bandaged hand, which he caught in a toy box about a week earlier.

By Sunday night, he was dead and his tiny body covered head to legs with 53 bruises and 17 abrasions.

One of the marks included a "smiley face" scar on his leg caused by a heated cigarette lighter at least four to six weeks earlier.

The court heard the first fatal blow to Tyrell's abdomen would have been inflicted at least a day prior, and the second within several hours of his death.

Scown testified at Strbak's hearing that Tyrell vomited multiple times on the Saturday and Sunday.

He said the boy should be taken to hospital, but Strbak refused and instead went in search of cannabis.

That evening while Tyrell was sleeping, Scown heard a gargling noise coming from his room and discovered vomit coming from the child's nose.

"(He has) gone all limp and it's like he's just not there," Scown told the triple-zero operator.

"Looks like he's gonna die on me."

Scown, 34, was sentenced to four years' jail after pleading guilty to manslaughter but walked free from court in October, having already spent two years and eight months in custody.

He was sentenced on the grounds he was not responsible for Tyrell's injuries but failed in his duty of care by not seeking medical help for the boy.

Justice Applegarth on Monday found there was a "no compelling evidence that Scown inflicted the fatal injuries" that weekend.