Opponents are mobilising against a bill that would allow gays and trans-gender couples to marry.
Labour MP Louisa Wall's bill has divided the church, and conservative groups have indicated they will attempt to build opposition against it.
Conservative Party leader Colin Craig said he was planning a "research-based" campaign on the advantages of heterosexual relationships and traditional family structures.
"I'm keen to be part of a campaign to get out there on this issue. It would look intelligently at the differences between homosexual parenting and a Mum and a Dad. Does gender matter, does role-modelling matter?
"As people become more informed, their opinions will crystallise."
Mr Craig, whose party received 2.65 per cent of the vote in the general election, has committed part of his personal wealth to battling social policy in the past, such as Sue Bradford's anti-smacking bill.
The millionaire property developer said he was willing to make another personal investment to fight gay marriage.
Ms Wall said she was expecting strong debate on the bill. But she also felt New Zealand had become more tolerant in the eight years since the civil union debate.
This was reflected in the change of heart by some MPs. Prime Minister John Key, Maori Party co-leader Tariana Turia and United Future leader Peter Dunne all voted against civil unions in 2004 but said they would support marriage equality at the first reading.
Ms Wall said: "I think it is a different New Zealand. But I don't underestimate that there are some people who think this will negatively affect their lives. Particularly religious institutions that think they will be compromised."
Ms Wall re-emphasised yesterday that her legislation would change the state's definition of marriage, not the church's definition, and religious institutions would be free to opt out of marrying same-sex couples.
The Catholic Church said it opposed the bill and was appealing to MPs to keep marriage "as defined as between a man and a woman".
The Bishop of Auckland, Pat Dunn, said the church affirmed love and fidelity in all relationships, but there were other legal avenues aside from marriage that allowed couples to "publicly cement their lifelong commitment to each other".
Several MPs said they would oppose the bill on similar grounds. National MPs Eric Roy and Mike Sabin said they felt that same-sex couples could already have their relationships recognised under law.
The Anglican Church resolved at its general assembly this month to begin a discussion about the nature of marriage, but a commission looking into same-sex blessings will not report back until 2014.
A Herald survey of MPs showed strong support for the bill to pass its early stages but also a lot of uncertainty in the National caucus. Maori Party co-leader Pita Sharples, Labour leader David Shearer and all Green MPs have indicated their support.
Votes so far
Are you for or against Louisa Wall's bill?
53 MPs for
3 MPs against
48 MPs undecided
17 MPs have not responded