Pre-schooler Tyrell Cobb suffered serious injuries, including a kick to the stomach, in the presence of his stepfather Matthew Scown, a court has been told.

Tyrell's mother Heidi Strbak is accused of delivering the fatal blow on the 4-year-old Gold Coast boy in 2009 but Brisbane's Supreme Court heard she did not witness a number of incidents that caused severe injuries.

Scown was in the witness box for most of Wednesday and spent more than two hours being grilled by defence barrister Greg McGuire during Strbak's sentencing hearing.

Heidi Strbak is facing a sentencing hearing over the death of her young son Tyrell Cobb. Photo / AAP
Heidi Strbak is facing a sentencing hearing over the death of her young son Tyrell Cobb. Photo / AAP

McGuire pointed out to Scown that on three occasions only he saw Tyrell suffer injuries and was home alone with the boy on another time. A fortnight before Tyrell died, Scown admitted he kicked the boy because he had urinated on the carpet.

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"The following weekend he watched a toy box lid jam one of the boy's fingers. On both occasions his mother was in another room," Mr McGuire said.

The day before his death, Tyrell fell forward on a staircase behind his mother's back and that night fell in shower while Strbak was absent. "Do you say it was an unfortunate coincidence that there were four occasions when you were alone with Tyrell ... to witness what happened?" McGuire asked Scown.

Scown said that Strbak was in their unit on the three occasions and Tyrell didn't cry.

The day after Tyrell fell in the shower he died.

Strbak has pleaded guilty to manslaughter on the basis of criminal negligence. The prosecution is arguing she was responsible for the blunt trauma blow to the abdomen that caused her son's death.

Scown earlier told the court Strbak had slapped her son so hard across the face on the weekend he died and that he was bruised under his eye.

"He vomited and she slapped him across the face. On the Sunday there was bruising under his eye," Scown told court while pointing to his left cheek. Mr McGuire questioned the strength of that evidence because he made a police statement in September.

Tyrell Cobb's body was covered in injuries.
Tyrell Cobb's body was covered in injuries.

"That's when you decided to give evidence against Heidi with a view to getting some benefit for yourself and that's where you describe to police that you saw her smash him across the face," McGuire told the court.

He replied: "That is the reason she didn't take him to the doctor."

Scown, who testified that Strbak was a daily "dope smoker" who would get impatient and argumentative without cannabis, was last month sentenced to four years' jail for the boy's manslaughter.

His sentence was immediately suspended after he'd served two years and eight months in custody.

The sentencing hearing before Justice Peter Applegarth continues today.