Kidman emerged from a luxurious harborside mansion to greet about 20 reporters and photographers who had brought flowers and sang "Happy Birthday" to her over the intercom.
"Thank you for the flowers and the singing," she said, before posing for photographs. "It's nice to be home."
Asked how she planned to spend her birthday, Kidman replied: "Not much, dinner at home with mum -- boring."
Kidman said she was embarrassed by all the attention.
Later, she sent bottles of water and a case of beer out to the media with an attached note saying: "Enjoy! Nicole and Keith," the Australian Associated Press reported.
Kidman and Urban are reportedly staying in separate residences in the days prior to their wedding, which is expected to be a black-tie event involving a tight circle of friends and family.
Guests have been told to keep their schedules free from Friday to Monday, as they will be given only 24 hours notice as to the time and location, local media said.
It will be Urban's first marriage, but the second for Kidman, who was divorced from Tom Cruise in 2001 after 10 years of marriage.
Her two children, Isabella, 13, and Connor, 11, are expected to arrive later this week, the Australian Associated Press reported Tuesday.
Kidman won an Oscar for her portrayal of Virginia Woolf in 2002's "The Hours." Urban, who is based in Nashville, Tenn., won a Grammy this year for best male country vocal performance for "You'll Think of Me."