Heath Ledger's father first discovered his son was dead when he turned on the television and saw the 28-year old actor's body being rolled out of his apartment on a stretcher.

At the Cutting Edge Addiction Conference yesterday, Kim Ledger spoke about his son's accidental overdose on prescription drugs in 2008.

Mr Ledger is now the patron of Scriptwise, an Australian not-for-profit organisation that aims to prevent prescription medication misuse and overdose fatalities.

Seeing Heath's death reported came after a shopkeeper stopped Mr Ledger to ask him if he was okay, but it was not until he saw the news on TV later that the question made sense.

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 Heath Ledger
Heath Ledger

A call from police in New York came four hours later.

He had trouble finding the time to grieve during those confusing first few hours.

"He was bulletproof. All young people are bulletproof."

At the time of his death, Heath had recently finished filming The Dark Knight and was working on The Imaginarium of Doctor Parnassus.

"It's difficult to trace and difficult to understand," Mr Ledger said.

"He was warned by his sister the night before but he still got caught out."

Getting rid of the stigma surrounding addiction-which was that the kind of people who suffered from it brought it on themselves and were not worth saving-was one of his goals in speaking at the conference, Mr Ledger said.

"We need to be compassionate with these people. It's a disease and you need to encourage these people to speak out."