Conservation Minister Chris Carter is marrying his long-term partner Peter Kaiser today in a civil union attended by Prime Minister Helen Clark.
The ceremony is understood to be taking place at the Waitakere City Council building, with 180 guests.
The Speaker of Parliament, Margaret Wilson, and Cabinet ministers are also attending, as are many of Mr Kaiser's family, who have made the trip from Holland. Mr Kaiser, the principal of Tirimoana Primary School in Te Atatu South, has Dutch ancestry.
Last night, Mr Carter said: "It is a chance to celebrate our relationship with our friends and family."
He was reluctant to go into details, saying it was a private occasion. "We didn't really want any publicity."
Both men will wear suits for the occasion.
Asked if they were going on a honeymoon, Mr Carter said he would be going back to Parliament "because like all Cabinet ministers I try to work as hard as I can".
Helen Clark said last night: "Chris and Peter have had a very long-term, loving relationship and I am pleased the law now enables them to have it recognised. I wish them continuing happiness."
The couple have been together for 33 years and their relationship has never been a secret.
They met in 1973 while working at the now-demolished Cornwall Hospital during the university and school holidays.
Mr Carter was 20, a third-year history student at Auckland University. Mr Kaiser was 16, a student at Kelston Boys High School.
Between them, they have three teenage children, who live as one household with their mothers.
Mr Carter was the first openly gay MP in Parliament and is now the first openly gay Cabinet minister.
In a posting on the gay website GayNZ.com, he says: "Next month, my partner Peter Kaiser and I will join the 700 couples who have formed a civil union since the law came into force in April 2005.
"It will be a special moment for us and a chance for our family and friends to give public witness to our 33-year relationship.
"We are fortunate to live in one of the most tolerant and respectful nations on the planet.
"In my role as a Cabinet minister I often meet GLBT [gay, lesbian, bisexual and transgender] and other progressive politicians from around the world.
"Many envy our strong record of progressive reform of the laws and regulations that had discriminated against GLBT New Zealanders."
Last year, Britain's Environment Minister, Ben Bradshaw, a friend of Mr Carter, became the first British MP to use his country's civil partnership legislation to marry his gay partner.
Not all politicians have accepted Mr Carter's sexuality. National's John Banks, then Minister of Tourism, called Mr Carter "Christine" and criticised the granting of travel perks to Mr Kaiser.