Marlin, the overprotective clown fish dad who isn't really funny, and his intrepid son, Nemo, are returning to Australian cinemas with a new underwater adventure to help their forgetful friend Dory find her parents.

Twelve years after the animated Oscar-winning Finding Nemo wrote Pixar's fortunes in box-office gold, the much-anticipated trailer for Finding Dory has been released, putting the good hearted Dory front and centre of the trio's new deep sea quest.

It's about a year after the original film. Marlin and Nemo are living on the coral reef with the Blue Tang fish who notoriously suffers short term memory loss.

The reef has triggered in Dory some disturbing memory of family.

"I remembered something," she says. "That's not possible, is it? Is it like a picture in your head and you think you've seen it before? I just used the word before which means I'm remembering. What was I talking about?"

Much of the new film is set in far away California at the Monterey Marine Life Institute, where Dory discovers she was born and freed into the ocean.

Ellen DeGeneres, who voices Dory and championed a sequel, premiered the trailer. On Monday DeGeneres also tweeted the first poster which bears the tagline, She just kept on swimming, a riff on Dory's optimistic catchphrase in Finding Nemo.

Recently, she tipped fans on Yahoo to the movie's central conservation theme: "I think that fish should be in the ocean. It's what this whole sequel is about: It's about rehabilitation and putting them back in the ocean. And we have to protect our oceans".

Finding Dory also features the voices of Albert Brooks as Marlin and William Dafoe as Gill, the escapee from the dentist's tank. There is, however, no sign of encore performances from Bruce the shark and Crush the turtle, character favourites of Finding Nemo.

Dory's parents are voiced by Diane Keaton and Eugene Levy. Modern Family co-stars Ed O'Neill and Ty Burrell will voice new sea characters, Hank the octopus and Bailey the beluga whale. Kaitlin Olson will play Destiny the whale shark. Dominic West and Idris Elba of The Wire join the voice cast.

Pixar reportedly changed the film's ending after seeing the 2013 documentary Blackfish, which detailed the inhuman treatment of Orca whales at the Seaworld marine park. Coincidentally, the trailer's debut debuted as SeaWorld in California announced it would phase out public displays by killer whales.

Returning as writer and director will be Andrew Stanton who piloted Finding Nemo to box office success. Stanton, who is joined by co-writer Angus MacLane, had to be persuaded to create the sequel.

Finding Dory will premiere in Australian cinemas in June 2016.