The Packed to the Rafters co-stars, who got engaged in October this year, said that they want to have a "small service" just for those closest to them.
Marais told The Daily Telegraph: "Whatever we do, I think it won't be big. It will be more a small service for friends and family. We'll keep it casual."
Stewart added: "I have an idea at the moment but it's still a secret and she can't know."
When asked if they would elope, Marais said: "Maybe, who knows. We're just enjoying being engaged and we don't really know yet what the year's schedule looks like.
"Everyone thinks we're being cagey and secretive but we have not had a minute to even think about planning a wedding yet."