Labour Party MP Louisa Wall is close to securing the support to legalise same-sex marriage, a Herald poll of MPs shows.
However, many MPs were refusing to commit to a decision on the marriage equality bill, and the final vote could come down to the wire.
A straw poll of all 121 MPs found that fifty-four MPs have indicated they would support it at least in its early stages, or were considering supporting it.
The bill, which was expected to come before Parliament in late August, needed 61 votes to pass.
New Zealand First this afternoon indicated that it might abstain from the vote, because it preferred to put the issue to a public referendum.
This would mean the bill would need 57 votes to pass.
New Zealand First leader Winston Peters said: "Serious issues like this should be decided by the public of this country and not a few temporary empowered politicians."
But he would not give a straight answer when asked to confirm whether his party would opt out of voting.
"We are not voting for it. We believe it should be by way of a public referendum."
All members of the Greens, the Maori Party, Mana, United Future and most of the Labour MPs have backed the bill.
If the remaining eight "undecided" Labour MPs backed their colleague's bill, it would pass.
National remained split on whether to vote for a change to marriage legislation.
Eight National MPs including Prime Minister John Key said they were in favour of same-sex marriage, but the vast majority of the party was undecided.
Most of the undecided group said they were waiting to properly read the bill, or were consulting with their constituents before making their mind up.