Country singer Sarah Buxton is a first-time mum.

The Outside My Window hitmaker and her musician husband Tom Bukovac welcomed a baby boy, Marshall, on 4 December (12).

Announcing the news in a post on Twitter.com, she writes, "Tom and I are in love with our sweet baby boy, Marshall! He came December 4th. My little Sagittarius."

Ironically Buxton, who has also penned hits for stars like Keith Urban and Big & Rich, went into labour soon after complaining about missing her due date.

Hours before giving birth on 4 December, she tweeted, "There is a baby doing the robot inside my womb. I don't think it knows or cares that its due date was the 1st. It's just groovin' in there."

The couple wed in 2010.