The 19-year-old actress, who played Ramsay Street local Rachel Kinski, will star as Ellie Linton, the heroine of the teen novels that have sold nearly two million copies in Australia and New Zealand.
"I was ecstatic!" Stasey told ninemsn. "I have a lot of respect for Ellie's character and feel privileged to be playing such an inspirational young female role."
Director Stuart Beattie praised his film's leading lady, saying: "Caitlin captured the perfect balance of intelligence, vulnerability and strength that is essential to this character."
Filming of Tomorrow: When The War Began is due to begin at the end of September in the New South Wales Hunter Valley.