Tonkin was born in Sydney. When she was four, she attended dance courses in classical ballet, hip hop, contemporary dance, and tap dance. At the age of 12, Tonkin began courses at the Australian Theatre for Young People (ATYP) at the Wharf Theatre. Tonkin attended Loreto Kirribilli and later graduated from the Queenwood School for Girls. In December 2005, Tonkin was cast as the role of Cleo Sertori on the Australian children television series H2O: Just Add Water which follows the lives of three teenage girls who turn into mermaids. Tonkin improved on her swimming abilities during the pre-production of the series. The series premiered on 7 July 2006 on Network Ten and has since gone on to be aired to a worldwide audience of more than 250 million. To promote the series she and her co-stars presented the best band award at the Nickelodeon UK Kids' Choice Awards in October 2007. In 2008, Tonkin was nominated for "Best Lead Actress in a Television Series" at the prestigious Australian Film Institute Awards. The series ran for three seasons with the series finale airing on 16 April 2010. Tonkin has also appeared on fellow Australian television shows Packed to the Rafters and Home and Away. In September 2010, Tonkin made her film debut starring in the Australian action ensemble film Tomorrow, When the War Began. In the film, which revolves around a group of teenagers waging a guerrilla war against an invading foreign power in their fictional hometown of Wirrawee, she played the role of the rich and uptight Fiona Maxwell. In December 2010, a sequel was announced with Tonkin expected to return, though production of a sequel never came to fruition. In January 2011, Tonkin moved to Los Angeles to pursue an international acting career. She was cast in March 2011 as Faye Chamberlain on The CW supernatural drama series The Secret Circle which premiered on 15 September 2011 to over 3.5 million viewers. The series follows a group of young witches who are a part of a secret coven. For her performance, Tonkin has received critical acclaim, with critics referring to her as the breakout star of the series; she was featured on Variety's list of "new faces to watch" and named one of 2011's breakout TV stars by E! Online. The Secret Circle, however, had only one full season as it was canceled on 11 May 2012. In August 2010, Tonkin was cast in the 3D horror film Bait 3D. Filming took place on Australia's Gold Coast and follows a group of strangers who are trapped in a supermarket after a freak tsunami with a pack of great white sharks. The film was expected to be released in Australia in September 2012. In August 2012, Tonkin joined the cast of The CW television series, The Vampire Diaries, in the recurring role of Hayley, a friend of Tyler. She joined her co-star from H2O: Just Add Water, Claire Holt, who portrays Rebekah Mikaelson on the show. On 11 January 2013, The CW confirmed that a spin-off series to The Vampire Diaries was in the works, titled The Originals. The series revolves around the Original Vampire family members, and Tonkin would "be heavily featured in the prospective pilot" of the series. The CW confirmed on 13 February that Claire Holt would also join the cast of The Originals, marking the third time Tonkin and Holt have starred together on a show. Tonkin has had a main role in the first three seasons. Tonkin has appeared in a number of advertisements which include Vauxhall Motors and was previously signed to Chic Management. Her modeling career includes shoots for Girlfriend, Teen Vogue, ELLE Australia, Miss Vogue Australia, Vogue Australia, Dolly and Free People. Tonkin has also appeared in Miles Fisher's music video "Don't Let Go". In 2012 Tonkin opened a website about health with friend Teresa Palmer called YourZenLife.