The Oscar-winning Kidman is set to marry Urban in a lavish wedding this weekend. Details have been carefully guarded, but local media have widely tipped Sunday night for the big event.
Watts — who was born in Britain but raised in Australia — is a close friend of Kidman, who she met when the pair were young actresses in Sydney. The "King Kong" star arrived on a flight from New York early Wednesday wearing a gray sweater and black pants, her hair pulled back with a purple head band.
Watts, who was nominated for an Oscar for her role in 2003's "21 Grams," is expected to attend a pre-wedding bachelorette party for Kidman later Wednesday at the 4 million Australian dollar (US$3 million; euro2.39 million) mansion of Kidman's sister, Antonia, local media reported.
"I'm very excited for my friend," Watts told reporters after saying she wasn't "at liberty to discuss" any details of the event.
Others reportedly on the guest list are Oscar winner Russell Crowe, "X-Men" star Hugh Jackman, media mogul Rupert Murdoch and his wife Wendi Deng, and Murdoch's Sydney-based son Lachlan and his wife, model Sarah O'Hare.
The two children Kidman adopted with former husband Tom Cruise — Isabella, 13, and Connor, 11 — are due to fly into Sydney later this week, local media reported.
Kidman celebrated her 39th birthday on Tuesday with a low-key dinner at her parents' home in the northern Sydney suburb of Greenwich. Kidman and Urban arrived in a black Aston Martin, and were greeted by a handful of fans who braved the southern hemisphere winter cold to catch a glimpse of the stars.
The pair are reportedly staying in separate residences in the lead up to the wedding — Kidman in the harborside mansion she formerly shared with Cruise and Urban at a downtown hotel.
It is Urban's first marriage, but the second for Kidman, who ended her 10-year union with Cruise in 2001.
Although she has kept details of her wedding strictly secret, Kidman's relations with the media have improved since last year, when she took two Sydney-based photographers to court for allegedly stalking her and planting a listening device outside her home. The case was eventually settled.
On Tuesday, Kidman emerged her home and briefly posed for photographs after a media pack sang "Happy Birthday" over the intercom. She later sent bottles of water and a case of beer out to the media group with an attached note saying: "Enjoy! Nicole and Keith."
Kidman — who was born in Hawaii but raised in Australia — won an Oscar for her role in the 2002 film, "The Hours." Urban, New Zealand-born but who also grew up in Australia, won a Grammy in 2005 for best male country vocal performance.