Cecil Patrick Kennedy was looking after Jordan Thompson alongside his own child at Singleton in the NSW Hunter region when the toddler's mother, Bernice Swales, went to the shops on March 19, 2005.
She returned an hour later and found Kennedy giving CPR to Jordan's unresponsive body.
He was rushed to Singleton Hospital and declared dead soon after.
More than 18 years later Kennedy, now 51, has pleaded not guilty to the toddler's manslaughter.
In a 2005 police interview shown to a NSW District Court jury on Monday, Kennedy said Jordan was sometimes sick when he returned from his father's care in the months before his death.
"He (usually) wouldn't stop crying. He'd cry for probably 30 seconds then laugh his head off then next thing he'd be breaking down crying again, then he'd spew all this black stuff out," he said in the video.
The jury heard Kennedy had told Ms Swales to take him to the hospital "but she never did", though she sometimes gave him Panadol.
On the day Jordan died, the toddler had appeared particularly ill.
"This time there were chunks and lumps coming out of his throat," Kennedy said.
When police searched his home, the only medicines found were two kinds of antidepressants and an antibiotic.
The 51-year-old told police in his 2005 interview he did not think it was possible Jordan could have gained access to the drugs himself.
"My tablets have always been up high," he said.
"I don't know how he got in there."
The initial investigation focused on a presumed accidental drowning, as Kennedy told his partner the child had slipped under the water when he was giving him a bath.
But autopsies found a toxic level of antidepressant in the toddler's blood - a medication Kennedy was using, which police found in the top shelf of his closet.
He also claimed the pharmaceuticals were usually kept in the kitchen cabinet and did not know when they had been moved to the wardrobe.
The trial will continue on Tuesday.
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