A historic first vote in favour of gay marriage is expected today, sparking sharp political and moral division.

Another Labour MP, Damien O'Connor, broke ranks with most of his caucus colleagues yesterday, suggesting Parliament should be focused on "far bigger issues" - and ACT leader John Banks made a U-turn on historical anti-gay comments to back the bill.

Other MPs ducked for cover, refusing any comment on how they would vote.

West Coast Tasman MP Damien O'Connor's is likely to vote against the bill.

"We need to uphold marriage as a structure, as a construct around family life as a best intent," Mr O'Connor said.

"There are far bigger issues in society, and the inequality across the board that's being driven by the National Government is something everyone should be concerned about and that's what we should be focusing on."

Mr O'Connor will join fellow Labour MPs Su'a William Sio and Ross Robertson in voting against the bill led by their caucus colleague Louisa Wall.

Rallies both for and against Ms Wall's Definition of Marriage Amendment Bill will turn out at Parliament today ahead of the first reading vote, which is expected late tonight.

Lobbyists about Parliament yesterday included a Korean delegation of pastors strongly against gay marriage.

"Marriage is created and ordained by God . . . as a unit between one man and one woman to reflect the image of God," the Rev Chang Bum Ko said.

He said he would return to Korea if the bill became law.

The pastors handed MPs a legal opinion by lawyer Ian Bassett, which was commissioned by the lobby group Family First.

Mr Bassett's opinion suggested church ministers, marriage celebrants and even wedding photographers who withheld their services to same-sex couples on the grounds of a moral objection to gay marriage would be breaking the law if Ms Wall's bill was adopted.

Family First delivered a petition to Parliament that it said included 50,000 signatures against gay marriage. "Same-sex couples already have legal recognition through civil unions, so there is no need to redefine marriage," its director, Bob McCoskrie, said.

But Ms Wall said there was "an inherent discrimination" against same-sex couples in the law.

Pastors would still be able to choose who they performed a marriage ceremony for, she said. "What I don't like is scaremongering."

Ms Wall will have the support of most Labour MPs, all Green MPs, Maori Party MPs and at least a dozen National MPs, including Prime Minister John Key, which should be enough for the bill to pass for consideration by a select committee.

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