Kidman dated the singer from 2003 to 2004, two years after the end of her marriage to actor Tom Cruise.
Meanwhile, Nicole previously admitted their relationship didn't last because the couple weren't "ready".
While talking about her highly-anticipated new HBO series Big Little Lies, Nic mentions she and costar Zoë Kravtiz already share a lengthy history.
"Well, I knew Zoe because I was engaged to her father", Kidman says bluntly. In 2007, she revealed that she had been secretly engaged, but didn't reveal the man who popped the question, telling Vanity Fair, "It just wasn't right. I love Lenny; he's a great guy", Kidman told Porter. Neither has commented on the rumours until now, when Nicole opened up about working with Lenny's daughter Zoe Kravitz in new TV show Big Little Lies, which she stars in and executive produces.
"[Nicole] can be quiet and shy, which is interesting because the beast of an actress that she is, is so loud and clear. I wasn't ready. We weren't ready". "After every take I wanted to just stop and say, 'Wow, '" Kravitz said.
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The Lion actress added that she and Kravitz were still on good terms. Instead, she offered some illumination on her attitude towards commitment and matrimony.
"I get engaged and I get married - that's my thing".
But their whirlwind romance quickly met its demise (luckily for Keith!) in December of the same year, after they spent Thanksgiving together at his Miami home. The story does not mention their engagement.
These days Nicole, 49, is happily married to country music star Keith Urban, who she tied the knot with in 2006. "It's all in the family", she said.