The Third Day, The Frost - John Marsden

And then there were five.... It has been six long months since Ellie and her friends returned home from a camping trip to find their families and friends imprisoned by an enemy that threatens to steal Australia's freedom. Only they can stop this. Like seasoned soldiers, their methods have become extreme, even involving terrorism. When she's not gathering food and supplies or running like prey to survive, Ellie wonders at what they've become: Are they now ruthless terrorists? The more involved and vicious it gets, the higher the stakes are raised. Everyone is fighting for their lives.