The 34-year-old Australian actor, who met Marais when they were cast as on-screen boyfriend and girlfriend Jake and Rachel in television series Packed To The Rafters, recently proposed to her with a diamond ring.
Stewart told Woman's Day magazine: "It's really bizarre to find the chemistry we have. Obviously she is super beautiful, but she is really smart and funny."
Marais's mother Karen said of the couple: "Jimmy [recently] produced this book of Yeats poetry he'd bought for Jessica, then two tickets to the Cuban ballet. They not only love the same music, but they sit down and play it together. He's a classical pianist and sings to her all the time."