"His sister was on the phone to him the night before telling him not to take the prescription medications with the sleeping tablets. He said: 'Katie, Katie, I'm fine. I know what I'm doing.' He would have had no idea."
The 28-year-old’s death was ruled an accidental overdose, with a fatal combination of oxycodone, diazepam, hydrocodone and doxylamine found in his system.
His father talked about how Heath felt pressure to perform, and says prescription medication is given up far too readily to people with a "high profile" such as his son.
"Because he was travelling a lot, he would pop in to a doctor. In the case of someone with a higher profile it's often a case of 'what do you want' instead of 'what do you need,'" added Ledger. "There's so much pressure on them to perform so even though your body is telling you that it's not good and needs time, it's like 'just take these painkillers and keep going'".
"That was the case with Heath. He had to be back on set to finish (the next day). They were doing night shoots in the freezing cold and he had a weak chest anyway. He'd caught this (cough) and just couldn't shake it but he thought he had to because he wanted to get the movie done."
Following Heath’s death, his father has worked to raise awareness around the misuse of prescription medication in Australia.
"A staggering number of people are killed in Australia by overdoses involving prescription medications," he told the Sydney Morning Herald.
Scriptwise has teamed up with Indivior to launch a new online resource for those battling opioid dependence as well as those around them called Turn To Help.
"It's a way to open discussion with family and friends, and your doctor without that fear of being judged," added Ledger. "It's very important that everyone knows there's a way through it."