Publicist Claire Raskind confirmed to the Sunday Star-Times Csokas had been cast in the role of Celeborn in Peter Jackson's $360 million film project.
In J R R Tolkien's classic trilogy the silver-haired Celeborn, King of the Elves, is married to Galadriel, to be played by Blanchett.
Csokas, a former music television presenter, played the bumbling Dr Leonard Dodds on Shortland Street. More recently he has been acting on stage, including a stint in Melbourne.
Csokas will be playing opposite the stunning Blanchett, 30, one of Hollywood's most in demand stars.
The Australian actress won a Golden Globe and was nominated for an Academy Award for best actress for her title role in Elizabeth. She can currently be seen on New Zealand screens in The Talented Mr Ripley opposite Matt Damon and Gwyneth Paltrow.
Csokas joins fellow former Street star Craig Parker in the Rings project. Parker is understood to have already filmed scenes for a small role.
Meanwhile, two other stars have slipped into Wellington to start work on Rings.
Raskind confirmed Australian actors Hugo Weaving and Mirando Otto had arrived in New Zealand. it is understood Weaving started filming his first scenes last week.
Weaving's film credits include The Matrix, with Keanu Reeves, but he is probably best known in this country for his role as Mitzi in The Adventures of Priscilla: Queen of the Desert, the Australian film about drag queens.
Weaving has been cast to play Elrond. he has been spotted at Festival 2000 performances with fellow star Elijah Wood.
Otto, who has just finished a film with Harrison Ford and Michelle Pfeiffer, also had a part in The Thin Red Line. She will play Eowyn.
Meanwhile, film distributors were reportedly impressed at the three-minute sneak Rings preview shown in the US recently. A mixture of edited footage and behind the scenes video images was screened at ShoWest, a presentation for powerful cinema ownders. "Everyone's really excited, it was the first peek the public had had," Raskin said.
It is understood former film critic Costa Botes is working for Jackson as a video diarist, filming the behind the scenes footage.
Raskind said studio bosses from New Line Cinema, the film's backers, had visited New Zealand with distributors.
Csokas was coy when contacted by the Sunday Star-Times yesterday. He accepted congratulations but said he could not comment without approval from Rings bosses.