The Perth actor was pronounced dead on Jan. 22 at the age of 28 of "acute intoxication" by mixing prescription drugs including oxycodone, hydrocodone, diazepam and others, according to the New York coroner's report. His death was deemed an accident.
His father said Ledger's reaction to his sister's concerns was, "I’ll be fine."
"It just put his whole system to sleep I guess,” Kim told the local site.
Kim is now an advocate for drug awareness and thinks his talented son was not an addict in any way.
"It was a one-off thing. That’s what killed us, because he was warned by his sister the night before: ‘You shouldn’t mix what you’re taking for pneumonia with your Ambien,’" he added. "He was a young guy that traveled all the time for work. Even as a 2-year-old, he hardly ever slept. He was trying to work and travel and do everything in a short space of time."
He continued, "Heath mixed a couple of drugs together with sleeping tablets and he’s gone forever."
Shortly after his death, Ledger received a posthumous Oscar for his role as The Joker in "The Dark Knight." That film is widely considered one of the best in the genre's history.
"Losing a child is something that never leaves you. You can live with the pain, but it never leaves you," Kim said, adding his work and mission is now to save other lives.