In a move that will surprise many, ACT MP John Banks has announced he will support legislation to legalise gay marriage at its first reading.
Louisa Wall's private member's bill has its first reading in parliament on Wednesday.
She believes the numbers are "pretty solid" for the bill to pass, with at least 61 votes needed in the 121-seat parliament.
MPs will have a conscience vote, and Mr Banks was widely expected to vote against the bill, despite his party's liberal beliefs.
The conservative former National MP voted against Homosexual Law Reform in 1986, calling its passage "a sad and sickening day for New Zealand".
However, on Tuesday he confirmed to media he will support the marriage equality bill at its first reading, but did not guarantee his support for its subsequent stages.
Asked why he was supporting it, Mr Banks said simply: "because I am".
ACT Party president Chris Simmons told NZ Newswire it was Mr Banks' own decision, and the party's board had not pressured him to support the bill.
Meanwhile, NZ First's eight MPs are refusing to say how they will vote, although MP Brendan Horan signalled they will oppose the bill.
He said the party's MPs "don't abstain on votes", repeating their call for a referendum on the issue.
"If there's no referendum in the bill, we won't support it. I won't be supporting it if there's no referendum in the bill."
At least 16 of Labour's 34 MPs are expected to vote yes.
Prime Minister John Key is supporting the bill and says he expects only about 12 or 13 of his caucus colleagues to do the same.
He is confident the bill will pass its first reading, but the votes could change as the legislation progresses.
All 14 Green MPs, the Maori Party's three MPs, Mana leader Hone Harawira and United Future leader Peter Dunne are also supporting the bill.