Public opposition to same-sex marriage has grown significantly since a law change to legalise it came before Parliament, a Herald DigiPoll survey shows.
Same-sex marriage campaigners blame scaremongering by religious groups for the increase in opposition, saying lobbying has intensified as the bill progresses through Parliament.
Opponents of gay marriage say the jump shows people are waking up to the negative social effects of changing the Marriage Act.
Asked what best fitted their view on marriage law, 48 per cent of those polled said marriage should remain between a man and a woman - an increase of 7.5 percentage points from a poll last June.
The number of people who supported a law change to allow same-sex couples to marry fell 4 percentage points in that time, but still outnumbered the opponents by a small margin.
The Marriage Amendment Bill is expected to return to Parliament tomorrow for the committee stages after easily passing its second reading by 77 votes to 44.
It could become law next month.
Labour MP Louisa Wall, the bill's sponsor, said the poll showed a majority of New Zealanders still in support of same-sex marriage, "despite the opposition spending what seems vast amounts of money on an active and negative campaign built on fear and misinformation".
The poll was taken in the week of the second reading of the bill, when lobbying from both sides of the debate was at its most intense.
Ms Wall noted the generational divide in the poll - two-thirds of over 65s opposed a law change, while young people overwhelmingly favoured it.
"It is young New Zealanders that this bill speaks to ... and when issues such as equality and discrimination are highlighted, where they stand on the issue is a solid foundation of support."
Aucklander Taryn Kerr, 25, said she was moved to speak out in favour of same-sex marriage because those who supported it considered the bill's passage a foregone conclusion while opponents were increasingly well-funded and vocal.
"Religious groups have really ramped their activism. I received a brochure the other day which was very sophisticated propaganda. The way they went about it, the wording, the imagery, was quite strong."
Ms Kerr, who is engaged to marry partner Sasha Kljakovic on Valentine's Day next year if the bill passed, was concerned about the polarised debate.
"A lot of people don't know gay people and have these ideas because they see people portrayed in certain ways. Until you see the full depth of breadth of somebody and know their stories you may be scared of difference simply because it's different to you. When in actual fact, we're exactly the same.
"We have burning red faces with hurt and anger because Sasha, my partner, is training her butt off to be a doctor. There's a certain irony in my partner training to save lives and yet those people won't give her a chance to live her life the way she wants to."
Family First lobby group founder Bob McCoskrie, who opposes same-sex marriage, said the poll backed his belief that the longer the law-change debate continued, the more public concern would escalate.
"We get past the slogans of equality and discrimination and start asking the bigger questions such as 'do we need to change the long-held definition of marriage or can we provide legal rights through the Civil Union Act?'."
He said claims his group and others had tipped the polls with scaremongering were "absolute baloney".
Religious groups have expressed fears that an amendment to the law does not protect religious ministers from breaching discrimination laws, and say not enough attention has been paid to changes to same-sex adoption law in the bill.
The debate over the legislation appeared to have solidified many New Zealanders' views, with nearly 98 per cent of those polled now prepared to take a firm stance on gay marriage.
Christian lobby group New Zealanders for Marriage intends to hold a vigil in Parliament grounds tomorrow night which it hoped would give politicians second thoughts about supporting the bill.
Prayers will be led by the Catholic Archbishop, John Dew, and other Christians, a statement from the group says.
"Changing the legal definition and therefore the meaning of the word marriage doesn't change what it has always been - a unique covenant between a man and a woman," the statement says.
"Opinion polls have shown that an increasing majority of the public do not support this change, and politicians are asserting their opinions over the will of the people in supporting this bill."
- Additional reporting by APNZ
* Source: Herald DigiPoll survey of 750 people, March 11-17. Margin of error 3.6 per cent.