Upon their return, they find the town they call home has been invaded, the beginning of a war against Australia.
As they escape the same fate of their friends and family, they retreat to the dangerous territory of their holiday grounds and start a guerilla war against the invaders.
First published in 1993, the series spanned seven books and has been called the greatest series for Australian teens of all time.
Heading up the cast of the film version is Caitlin Stasey (formerly of Neighbours) as Ellie.
Lincoln Lewis (Home & Away) Phoebe Tonkin (H20: Just Add Water), Rachel Hurd-Wood (Peter Pan), Deniz Akdeniz, Ashleigh Cummings, Chris Pang and Andrew Ryan also star in Tomorrow, When The War Began.