The New York City Medical Examiner's Office announced that the actor suffered "acute intoxication'' from the combined effects of oxycodone, hydrocodone, diazepam, temazepam, alprazolam and doxylamine.
The Brokeback Mountain star was found dead at his Manhattan apartment on January 22. He was naked in his bed, with a variety of sleeping pills nearby. Anti-anxiety drugs were also found in the apartment.
His ex-fiancée Michelle Williams arrived in his home city of Perth with the couple's two-year-old daughter earlier today to prepare for his funeral.