Up to 200 people are thought to have been invited to the celebration, which is being held in the town where Ladyhawke, real name Pip Brown, was born and raised.
Given the couple's affiliation with the arts, it is expected that a number of the country's musicians and actors will be there.
A source, who did not want to be named, told the Herald it would be a big day for the couple.
"There'll be over 200 people there. It's going to be quite big and lots of celebs [around]."
Over the years, the couple -- particularly Brown -- have been quite private about their relationship, but have a home together in Los Angeles.
The singer is known for her hits including My Delirium and Paris is Burning.
She is known for her love of men's clothing and regularly performs in men's jackets and shirts.
The source said no one was expecting her to be wearing a dress today.
"She'll be wearing the trousers, apparently. She never wears dresses."
Actress Sami grew up in Auckland and has had roles in Sione's Wedding, Outrageous Fortune and Top of the Lake.
She is also an accomplished comedian and sings in a band alongside her sisters Anji and Priya as The Sami Sisters.
Yesterday, Priya posted a photo of a polaroid picture on her social networking websites showing the bride-to-be -- as well as their mother and another family friend -- getting a pedicure at a nail spa. Priya wrote: "The event 2015."
The ceremony is thought to be going ahead at Te Parae Homestead wedding and private function centre.
The couple can look forward to a stunning day today, with the forecast in Masterton being sunshine all day with a high of 28C.
By Vaimoana Tapaleao of the New Zealand Herald
