Stepdad 'killed six-month-old after mistaking him for a spider

A man charged with killing his girlfriend's 6-month-old baby claims he mistook her son for a spider on his chest.

Melbourne man Dwayne Lindsey told a court that the baby rolled off his chest and hit his head against a heater after he jumped up after getting a fright.

He denied he had intended to kill the boy and is on trial in the Supreme Court of Victoria for murder.

Lindsey said he let out a "long dingo howl" after realising something was wrong with Chayse Dearing, who suffered severe traumatic brain and spinal chord injuries before his death on June 26, 2016.

The man accused of killing Chayse Dearing, pictured, says he mistook him for a spider on his chest. Photo / 7 News

Prosecutor Nicholas Papas QC reportedly said Lindsey's version of events did not match medical reports, which suggested Chayse's death was unlikely to have been accidental.

It is also alleged that Lindsey later told a witness he had, in fact, struck and kicked the baby.

The boy's mother, Michelle Dearing, had left Lindsey alone with Chayse after going on a 24-hour shopping trip with friends at 4am on the day of the boy's death.

Chayse's mother Michelle had left him alone with her boyfriend Dwayne Lindsay while she went shopping. Photo / 7 News

She said Lindsey called at 8.21am saying something was wrong with Chayse, that he wasn't breathing and that blood was coming from his nose.

Another woman who was shopping with Dearing at the time she received the devastating phone call told the court how her friend broke down.

"I heard Michelle screaming my name and I knew something was wrong," Melissa Jones said. "She was a mess."

Jones said she called the police and returned home with Dearing and another woman to find "medics working on Chayse".

"Michelle collapsed in the doorway of my bedroom," said Jones, who claims the young boy had a "horrible scream". "She was in shock basically."

A neighbour also claimed he heard Lindsey and Dearing arguing on the night of the tot's death.

Dwayne Lindsey claims baby Chayse's injuries were caused by accident. Photo / Police

Dearing told the court that the couple were planning to get married before the fateful night.

She said Lindsey had proposed to her just nights before Chayse's death.

Defence barrister Scott Johns said Lindsey did not intend to kill the child.

"Whatever you think of Dwayne Lindsey, he's not a murderer," he said.

"None of his actions came from an intention to cause any harm at all."

The trial continues.