Byrne was cast in her first film role, Dallas Doll, when she was 12 years old. She has appeared in a variety of Australian television shows including Heartbreak High, Echo Point, and the film Two Hands alongside Heath Ledger. She appeared in The Date, My Mother Frank, and Clara Law's The Goddess of 1967 for which she obtained the Female Volpi Cup at the 2000 Venice Film Festival. Meanwhile, she appeared as a guest in an episode of the cop drama series Murder Call. She also acted on stage, playing a lead role in La Dispute and starred in a production of Anton Chekhov's classic Three Sisters at the Sydney Theatre Company.

Byrne appeared in the music video of Darren Hayes's single I Miss You and starred with Australian musician Alex Lloyd in the music video for his single Black The Sun and was featured on the cover artwork for the EP. She also appeared in the TV Commercial for Sony and reunited with Alex Lloyd, appearing in his music video for 1000 Miles from the album Distant Light. In 2002, Byrne entered Hollywood with a small role as Dormé, the loyal handmaiden to Natalie Portman's Senator Padmé Amidala, in George Lucas' Star Wars: Episode II - Attack of the Clones and appeared the same year in the movie City of Ghosts with Matt Dillon. The year previously she had flown to the UK to shoot I Capture the Castle, Tim Fywell's adaptation of the 1948 novel of the same title by Dodie Smith. In the 2003 release, which relates the adventures of the eccentric Mortmain family struggling to survive in a decaying English castle in the 1930s, she portrayed Rose Mortmain, the elder sister of Romola Garai's Cassandra. In 2003 she also starred in three Australian films: The Night We Called It a Day alongside Melanie Griffith and Dennis Hopper; The Rage in Placid Lake for which she was named Best Actress at the Australian Film Institute with singer Ben Lee; and Take Away another comedy.

In 2004, Byrne starred as Briseis the Trojan priestess who was abducted during the Trojan War by Achilles (played by Brad Pitt), in Wolfgang Petersen's epic Troy, also starring Eric Bana, Peter O’Toole, Sean Bean, and Orlando Bloom. She then reunited with Peter O'Toole in the acclaimed BBC TV drama Casanova. Byrne appeared with Snoop Dogg in Danny Green's film The Tenants, based on Bernard Malamud's novel, and starred with Josh Hartnett and Diane Kruger in the romantic psychological thriller Wicker Park where she played Alex, the woman who manipulated Josh Hartnett's character to keep him apart from the woman he falls in love with. In 2006 Byrne portrayed Gabrielle de Polastron, duchesse de Polignac a French aristocrat and friend of Marie Antoinette, in Sofia Coppola's Marie Antoinette, alongside Kirsten Dunst and The Dead Girl directed by Karen Moncrieff. She and Marie Antoinette director Sofia Coppola have both played handmaidens in the Star Wars prequels: Coppola appeared in The Phantom Menace, Byrne in Attack of the Clones.

In 2007, she played Cassie, the pilot in Danny Boyle's science fiction suspense film Sunshine, Scarlett Ross, an army medical officer in Juan Carlos Fresnadillo's sci-fi horror 28 Weeks Later, the sequel to Boyle's 28 Days Later and appeared in the independent film Just Buried, a Canadian dark comedy written and directed by Chaz Thorne. Byrne is currently in a FX drama production Damages, playing the regular lead role of Ellen Parsons, a young attorney torn between her hard-hitting, high-stakes new boss (Glenn Close) and her own ambitions. She appeared in the Australian film noir The Tender Hook with Hugo Weaving.

Byrne was the face of Max Factor between 2004 and 2006 and named in the Most Beautiful People of 2007 list in Who Magazine. Byrne has supported UNICEF Australia by being the face of the 2007 Designers United campaign and a member of tropfest jury in 2006 and tropfest in 2007. She is a graduate and ambassador for NIDA's (National Institute of Dramatic Art) Young Actors Studio. She was recently named the first patron of Chauvel Cinemas presented by the Brisbane International Film Festival and named in honour of Charles Chauvel.

• Knowing (2009) (post-production) .... Diana Whelan
  
• "Damages" .... Ellen Parsons (16 episodes, 2007-2009)
       - I Knew Your Pig (2009) TV episode .... Ellen Parsons
       - Burn It, Shred It, I Don't Care. (2009) TV episode .... Ellen Parsons
       - I Lied, Too. (2009) TV episode .... Ellen Parsons
       - Because I Know Patty (2007) TV episode .... Ellen Parsons
       - There's No 'We' Anymore (2007) TV episode .... Ellen Parsons
        (11 more)
• Adam (2009) .... Beth
• The Tender Hook (2008) .... Iris
• Just Buried (2007) .... Roberta Knickel
• 28 Weeks Later (2007) .... Scarlet
  ... aka 28 semanas después (Spain)
• Sunshine (2007) .... Cassie
• The Dead Girl (2006) .... Leah
• Marie Antoinette (2006) .... Duchesse de Polignac
  ... aka Marie Antoinette (France)
• The Tenants (2005) .... Irene Bell
  ... aka Die Letzten Mieter (Germany: TV title)
  ... aka Room 109 (Germany: DVD title)
• "Casanova" .... Edith (3 episodes, 2005)
       - Episode #1.3 (2005) TV episode .... Edith
       - Episode #1.2 (2005) TV episode .... Edith
       - Episode #1.1 (2005) TV episode .... Edith
• Wicker Park (2004) .... Alex
• Troy (2004) .... Briseis
• Take Away (2003) .... Sonja Stilano
• The Rage in Placid Lake (2003) .... Gemma Taylor
• The Night We Called It a Day (2003) .... Audrey Appleby
  ... aka All the Way (USA: DVD title)
  ... aka All the Way: The Kidnapping of a Music Legend (USA: DVD title)
• I Capture the Castle (2003) .... Rose Mortmain
• City of Ghosts (2002) .... Sabrina
• Star Wars: Episode II - Attack of the Clones (2002) .... Dormé
  ... aka Attack of the Clones (USA: short title)
  ... aka Attack of the Clones: The IMAX Experience (USA: IMAX version (promotional title))
  ... aka Star Wars II (USA: promotional abbreviation)
  ... aka Star Wars II: Attack of the Clones (USA: video box title)
  ... aka Star Wars: Episode II (Australia: TV title)
• The Pitch (2001) .... Girl