Country singer Keith Urban paid a heart-warming tribute to his wife, Nicole Kidman, during his concert in Sydney on Saturday praising her as ''extraordinary, pure and a beautiful light.''

The 46-year-old country music star, who has previously credited his wife with helping him overcome his addictions, gushed about the Grace of Monaco actress before performing the song he wrote for her, Once In A Lifetime, during his concert in Sydney on Saturday (June 21).

Speaking to the crowd Urban, who has daughters Sunday, five, and Faith, three, with Kidman, said: ''I want to make a special dedication, about this time eight years ago we were about five days away from getting married.

''A few days before that my fiancée and I were talking about the wedding and she was worried about getting married to me - understandably - I was trying every way I could to reassure her that I was born to be her man ... the only thing I could do was reassure her, comfort her, be strong.

''The next day, I happened to be with a friend of mine who was a songwriter and I was telling him what happened and he said we should write about that so I wrote this song for my wife and she's here tonight.

''In a world of so much darkness and toxicity and scary people, she is extraordinary, pure and a beautiful light and I love you with all my heart baby girl.''

Kidman, 47, was in the crowd alongside her parents.