In December 2005, Tonkin was cast in the role of Cleo Sertori on the Australian children’s 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.