Same-sex marriage has become a near-certainty after legislation to allow it passed its first reading last night by a two-to-one margin.
The size of the majority means the Marriage (Definition of Marriage) Amendment Bill is now expected to pass comfortably into law.
Huge cheers from a full parliamentary chamber greeted the historic vote, in which 80 MPs favoured a law change and 40 opposed it.
The bill's sponsor, Labour MP Louisa Wall, said she had been hoping for 61 votes - the least needed for a majority in the 121-seat house. To get 80 was "very special".
The vote was originally declared at 78 in favour, but this was updated to 80 when it was found that National had forgotten to cast proxy votes for coalition MPs John Banks of Act and Peter Dunne of United Future.
Earlier, Ms Wall told the House: "Today is the time to open the institution of marriage to all people who are eligible. There is no reasonable ground on which the state should deny any citizen the right to enter the institution of marriage if he or she chooses."
The Labour MP for Mangere, Su'a William Sio, was the first to speak against the bill, saying he would oppose the legislation on the behalf of his constituents.
"This is a matter that is very sensitive for members of my constituency - within the Pacific and faith community, even within my own family."
Mr Sio was one of only three Labour MPs to vote against the bill.
The most dramatic turnaround of the night came from National's Hunua MP, Paul Hutchison, who had told the Herald on Tuesday that he would oppose the measure.
After revealing that Ms Wall had given him last-minute advice on same-sex marriage, he said: "I cannot construct a strong enough intellectual, moral, health or even spiritual argument against it."
Ms Wall described his u-turn as the most poignant moment of the debate.
She said Dr Hutchison had called her late on Tuesday night.
"He wanted to engage, he wanted to have a conversation and had some questions and I was able to provide some evidence about those questions."
Dr Hutchison, a former obstetrician and gynaecologist, said he was deeply concerned that gay adolescents had a suicide rate five to eight times that of heterosexual adolescents "in a country that already has an appallingly high suicide rate".
He was told by gay MPs Maryan Street and Kevin Hague that the passage of the bill would have a profound effect on the marginalisation that adolescents felt.
"From a health perspective, we should be doing everything possible to create an environment in New Zealand where everyone feels they are included," Dr Hutchison said.
New Zealand First leader Winston Peters said his party wanted to put the questions of same-sex marriage to a referendum.
The party had proposed an amendment to Ms Wall that would require a binding referendum on the bill's passage, but that was rejected because a poll suggested that 60 per cent of the public favoured gay marriage.
Ms Wall also moved to reassure churches that their definition of marriage would not be altered by the legislation, and said that ministers would not be obliged to marry couples against their will.
Catholic Bishops voiced their disappointment after the bill passed its first reading in Parliament.
In a statement after the vote last night, NZ Catholic Bishops reaffirmed their opposition to the bill.
"The Catholic Church affirms love, fidelity and commitment in all relationships, but believes that marriage should be defined as being between a man and a woman," the President of the New Zealand Catholic Bishops Conference, Archbishop John Dew said.
"To propose any alternative definition will have implications in law, and in society, but also for education and the family structure which throughout history has been seen as the fundamental unit in every society."
The bishops said there are already legal avenues for same sex couples to publicly declare their love and commitment to each other.
"Society doesn't have the right to deprive a child of both its father and mother, both equally significant in their upbringing. We're concerned about children growing up without one or both parents as part of the primary parenting partnership. We also understand that as humans we have a real need to get to know both of our biological parents," Archbishop Dew said.
"While there are families which include single parents and same sex couples raising children there is a question to be asked about whether we want to legislate for a new norm for the family unit."
Labour MP David Clark, a Presbyterian minister, said there was an absence of advice for gay marriage in the Christian scriptures, particularly in Jesus' words.
"I suspect he would say that marriage is frequently paraded in the media by those who claim a Christian viewpoint as really a thinly veiled defence of Victorian morality."
Mr Clark voted for the bill.
Labour MP Raymond Huo, who had earlier said he was leaning towards supporting the bill, did not cast a vote because he felt the Chinese community was overwhelmingly opposed to it.
Mr Huo said he would review his decision at the select committee stage.
- Additional reporting: Kate Shuttleworth, Paul Harper