Nicole Kidman says she's "deeply in love" with her new husband country music star Keith Urban, but stops short of describing their marriage as "happy".

The pair married in Sydney in June of last year but their newlywed bliss was short lived, with Urban checking into rehab for a 90 day stint just four months later.

The actress says that despite her strong feelings, love is no guarantee that the relationship will last.

She told US magazine Interview: "You never know where something's going."

"My husband and I are committed to each other and deeply in love. That's how I would put it. We are working on staying in that place and hopefully we will for the rest of our lives."

Told by the magazine interviewer that she and Urban appeared happy together, she replied: "I don't ever say that."

"Partly because interviews are done in advance, and you never know where something's going.

"People's lives together are complicated and beautiful and that's what they should stay. They're very quiet. Complicatedly quiet. They don't need to be broadcast."