The former Packed to the Rafters co-stars, who got engaged last year, confirmed the pregnancy after various rumours that Marais had been spotted with a baby bump.
A statement from their representative to The Daily Telegraph read: "James Stewart and Jessica Marais announce they are expecting their first child. The couple are excited and looking forward to this next phase of their lives."
Marais quit Packed to the Rafters last year in order to embark on a Hollywood career and soon landed a role in The Magic City alongside Grey's Anatomy actor Jeffrey Dean Morgan.
The Logie Award-nominated actress recently revealed that her fiancé "can't wait" to have children, but she said: "I'm happy. So long as I am living my life as a good person and have my family and Jimmy around me, I'm happy no matter what."
The pair have previously said that their wedding "won't be big" because they want a "small service for friends and family" and to "keep it casual".
Earlier this year, Marais and Stewart were plagued with split rumours but dismissed them as rubbish.
> Jessica Marais: 'I'm not pressured to be thin'
> Jessica Marais reveals trauma of watching her father die