Ledger's family returned to Australia from the US this week and are planning a private funeral service for the actor.
The actor is best known for playing a gay cowboy in the western film Brokeback Mountain, a performance that earned him an Oscar nomination in 2006.
News Ltd newspapers reported that Ledger's former girlfriend, Michelle Williams, and their daughter, Matilda, arrived in Perth on Wednesday for the ceremony.
In a statement released on Wednesday, Kim Ledger said the toxicology results should "put an end to speculation" about his son's cause of death.
|Ledger's sudden death in the US sparked grief|
all the way to Australia [GALLO/GETTY]
"While no medications were taken in excess, we learned today the combination of doctor-prescribed drugs proved lethal for our boy," he said.
"Heath's accidental death serves as a caution to the hidden dangers of combining prescription medication, even at low dosage."
"Few can understand the hollow, wrenching, and enduring agony parents silently suffer when a child predeceases them."
Following Ledger's death, Kevin Rudd, the Australian prime minister, said his death was a tragic loss of "one of our nation's finest actors in the prime of his life".