The wentworth website states, "Sonia is the wealthy, self-made dynamo behind a cosmetics empire that was owned and run by her late husband, Geoffrey Stevens.
And while Geoffrey was something of a dabbler, Sonia was passionate, hard working and always had the ability to focus on a goal, no matter how elusive it may first seem.
"[1] A representative said about Thornton's casting as Sonia "“Having Sigrid Thornton sign on to return behind the walls of Wentworth is a casting dream and a wonderful acknowledgement of the work the writing team has put into the creation of Sonia Stevens and our plans for her and all our characters.
[3] Sonia first appears in Season 4, Episode 6 'Divide and Conquer', having been remanded at Wentworth pending trial for the murder of her friend, Helen Masters.
Don Kaplan grows impatient and advises Liz to lie under oath in court.
Flashbacks reveal that Sonia had in fact murdered Helen, who had attempted to blackmail her over the mysterious death of her husband.