The drama stars Britt Robertson as a young girl called Cassie who moves to Salem and discovers that she is a witch.
Deadline says that Tonkin has now signed up to play Faye, one of the members of Cassie's coven.
Tonkin previously appeared in some episodes of the Australian soap Home And Away and starred in the movie Tomorrow, When The War Began.
Secret Circle, which is based on a book series by LJ Smith, is being executive produced by The Vampire Diaries showrunner Kevin Williamson.
> Thomas Dekker boards 'Secret Circle'