Heath Ledger's sisters have denied a long-held assumption about his death.

After a documentary celebrating Heath Ledger premiered at the Tribeca Film Festival on Sunday, two of the actor’s sisters, Kate Ledger and Ashleigh Bell, and his long-time friend Matt Amato got up on stage for a rare public interview.

At the time of his death in 2008, Ledger was 28 and at the peak of his career after an impressive turn as The Joker in The Dark Knight.

Since then, his accidental overdose — thanks to a fatal combination of sleeping pills and prescription medications — has largely been blamed on an ill-fated attempt to manage his mental demons, which many believed were exacerbated by the demanding role and the toll it took on his historically fragile mental health.

Those who knew him believe otherwise and want I Am Heath Ledger to bust the myths surrounding the talented young Australian’s death.

“I hope it’s an antidote to a lot of the gossip that exists in the world,” Amato told the Tribeca audience, according to The Daily Beast.

Source: Getty

“There are some really terrible things about Heath out there.”

His sisters were equally baffled by the rumours, as Bell explained:

"I don’t know if it was the case that we wanted to clear it up, but as soon as he had the movie everything that came into light about The Joker, we were all so confused.

“It was coming out that he was depressed and it was taking a toll, and we were going, ‘What?’”

Meanwhile, Ledger said she was "shocked", because for her at least, her brother's performance was him in his element.

Heath Ledger as The Joker in 'The Dark Knight'.

"He had an amazing sense of humor, and I guess only his close family and friends really knew that. But he was having fun. He wasn’t depressed about The Joker,” she said.

The film itself has already been praised for celebrating Heath as a performer, filmmaker, father, and friend through interviews with those who knew and worked with him, as well as previously unseen footage shot by the actor himself.

The mother of his daughter, Matilda, Michelle Williams does not appear in the biopic but gave the project, which will have limited cinematic release, her blessing.

Over the years, the Ledger family has had multiple approaches to make such a film, but waited until they felt comfortable.

If you think you may be experiencing depression or another mental health problem, please contact your general practitioner or in Australia, contact Lifeline 13 11 14 for support or beyondblue 1300 22 4636.