Meanwhile, TVNZ weatherman Tamati Coffey is also marrying his longtime partner, Tim Smith, in the lead-up to the New Year.
Coffey is having a "My Big Fat Gay Wedding" theme at The Wharf, on Auckland's North Shore, according to a source close to the couple.
"Apparently his bridesmaids are fighting about their outfits," said the source.
Coffey tweeted to his legion of fans last week: "Fyi team. I'm on an early xmas. Got a gig at the end of this month that I need to put some effort into. So, not on air, but still tweeting!"
Celebrity agent Andy Haden, who represents Williams, was not keen to talk about the couple's plans.
Guests have been given strict instructions not to take photographs.
Flowers will be supplied by Queenstown-based wedding florist The Flower Room, which specialises in vintage style bouquets.
Staff were tight-lipped when asked about details. "Yes, but I can't comment on that, sorry. We have been given very strict instructions ... I'm sorry."
The couple had been together for seven years when Williams proposed to Green,
who grew up on the West Coast.
In a previous interview she told how she had hardly worn a dress before meeting Williams.
The guest list will include teammate and business partner Richie McCaw, who turned down a knighthood this week.
Queenstown wedding caterer Brent Makeham said those involved with celebrity weddings were usually made to sign confidentiality contracts.
"We circle the wagons when need be. Only a certain number of people know what's happening. The staff just know there's a wedding on and turn up. On the day they get a surprise," he said.
"If we were [involved] I'd have to say no comment."
Makeham's company catered for the wedding of Andrew Hore, a teammate of William's, three years ago.
"With Andrew, they wanted to keep it low key and they did. Wanaka's that sort of place. It's a small town and small towns can keep secrets. "
Also getting married over summer is new 60 Minutes reporter Guyon Espiner, 40, the current TVNZ political editor, to Emma Wehipeihana, 26, in Golden Bay in February.