Heath Ledger's highly anticipated final completed film, The Dark Knight, opens across Australia on Wednesday.

Ledger, who died of an accidental drug overdose in New York in January aged 28, stars as comic-book hero Batman's chilling arch-enemy, The Joker.

Momentum for the Australian actor's creepy rendition of the role to be nominated for a rare posthumous Academy Award is growing.

The movie had its world premiere in New York on Tuesday that was attended by Ledger's family - his father Kim, sister Kate and mother Sally Bell.

"The Dark Knight is everything we hoped it would be and more," Kim Ledger said after the premiere.

"Heath loved the experience of creating this character and working on the film. We are so proud of our boy."

And his co-stars heaped praise on Ledger's performance.

British actor Michael Caine, who plays Batman/Bruce Wayne's butler Alfred, repeated his belief that at the very least Ledger should receive a best supporting actor nomination.

"My favourite part of the film is the opening monologue of Heath Ledger as the Joker, it's one of the best close-ups I've ever seen in my life," Caine said on the red carpet.

"He was a young actor and this is his moment. He's going to get the Oscar for this."

The end credits of The Dark Knight include a farewell note to Ledger, along with special-effects technician Conway Wickliffe, who was killed last September in a stunt-car accident.

"In memory of our friends Heath Ledger & Conway Wickliffe," reads the tribute included in the credits.