"It was my [38th] birthday, and he stood outside with gardenias at 5 a.m. on my stoop in New York," Kidman, 51, tells People exclusively in this week's cover story.
"That is when I went, 'This is the man I hope I get to marry.'"
Urban, 51, took her to Woodstock, N.Y., the site of the 1969 music festival, on a motorbike for a getaway ("My kind of guy!" she says), and she's never looked back.
"It was pretty intense," says Kidman, whose gritty drama Destroyer will be released wide this month. "I believed by that point he was the love of my life. Maybe that's because I am deeply romantic, or I'm an actress, or I have strong faith as well, but I just believed, 'Oh, okay, here he is.'"
For his part, Urban was always equally smitten with his Oscar-winning bride. He tells People, "I could sum up Nic as a wife, mother and human being in three words: She's all heart!"