In recent months, fans have seen the actress's alter ego stage a sham reunion with ex-boyfriend Jarrod 'Toadie' Rebecchi (Ryan Moloney) in a bid to prevent her loved ones from finding out that Dan Fitzgerald is the father of her child.
However, speaking to TV Week, Bonner teased that Steph and Toadie's secret will be uncovered by a mystery character later this year.
The actress said: "Someone Toadie has been involved with in the past sets out to investigate him and uncovers more information than they ever hoped for.
"It's the beginning of the secret spreading - with a few surprises along the way!"
Bonner recently announced that she is to leave her role as Steph later this year. Producers have promised an exit storyline that will "do the character proud".