Since her breakthrough role in the hit Australian comedy “Muriel’s Wedding” in 1994, Toni Collette has managed, every few years, to be in the big movie everyone was talking about. In “The Sixth Sense” (1999), she played Haley Joel Osment’s mother and received an Oscar nomination for Best Supporting Actress. In “The Hours,” she played Julianne Moore’s neighbor and while the other actors in the film were sometimes overacting with a capital A, Collette made her mark in just a few scenes as a cancer-stricken neighbor. Most recently, she was part of the ensemble cast in “Little Miss Sunshine” – the Oscar-winning blockbuster that became the template for the feel-good movie every Hollywood studio was looking.

Collette, 36, says she doesn’t have a formula for success in picking projects except to say, “I go with my gut. I don’t overthink things.”

Along with drama school buddy Cate Blanchett, Russell Crowe, Rachel Griffiths and Nicole Kidman, she is part of the “Aussie posse” that has found staggering success in this country. Collette is the star of “The United States of Tara,” a suburban black comedy about a traumatized midwestern mother afflicted with Dissociative Identity Disorder (DID). The Showtime series, Collette’s first, is “Sybil” with laughs and the actress says the part was too good to pass up.

“I never had any great aspirations to work in TV, but when I read the script, I loved it so much, I said, ‘Who gives a shit how big the screen is? I just want to get in there and be this person and explore this situation because it’s so fascinating.”

In addition to Tara Gregson, mother of two teenagers written in the self-conscious overly ironic “Juno” mode – the creator of the series is that movie’s Oscar-winning screen writer Diablo Cody – and wife to a mellow landscape architect, Collette plays three other roles. Referred to by the characters as Tara’s “alters,” they are, in Collette’s words, “what’s missing in Tara, what she can’t tap into.”

T is a potty-mouthed, predatory teenager who’s “just giving the finger to the world and living in that land of teenage responsibility.” Alice is Betty Crocker on crack – a perfect housewife who, in the second episode’s best scene, washes out Tara’s daughter’s mouth with soap in a restaurant bathroom. Buck is a butch biker dude with a mean right hook – and Collette’s greatest challenge here.

“Who wouldn’t to play a guy? It’s not often that you get into a fistfight with a male actor. It’s a good time,” she says. More seriously, she adds, “Buck comes forward like a bouncer. He’s aggressive as he protects. Alice is about control. She’s so anal I get a sick kick out of it. T is about escapism.”

Behind the multiple personalities, which the family manages to deal with as smoothly as can be expected, lurks a traumatic experience that has led Tara to split off into T, Buck and Alice. Collette promises that we will find fairly soon -by episode 4-what lies at the root of her character’s problems.

“That’s part of the arc, so to speak,” she says. “The show’s all about acceptance. It goes beyond focusing on the illness. Doesn’t make a big deal about it.”

While on location in Los Angeles to shoot “Tara”‘s 12 episodes, Collette travelled from Sydney with her family, husband David Galafasssi and 1-year-old daughter, Sage Florence. The show is set in Overland Park, Kansas, and, like most Australian actors, Collette aced the accent. She says that she never studied accents while briefly attending the National Insitute of Dramatic Arts (she left after 18 months to make her first film, “Spotsiwood,” with Russell Crowe and Anthony Hopkins) but honed her ear by watching imported American TV.

“I’m grateful to all my bad TV watching as a child,” she says. “I wouldn’t want to be focusing on my accent while I’m in the middle of a scene because it would be so distracting.”

THE UNITED STATES OF TARA

Sunday, 10 p.m., Showtime