MPs from the Labour Party's South Auckland strongholds are divided over a bill which would legalise same-sex marriage, with two concerned it could seriously hurt the party's support in its heartland electorates.

Labour MP for Mangere Su'a William Sio said he would oppose fellow MP Louisa Wall's bill when it came up for a conscience vote, and called for it to be withdrawn.

Mangere was one of three crucial Labour electorates in South Auckland, and he felt the wide opposition from constituents to the bill, particularly from Pacific Islanders, could cost the party at the next election.

Labour MP for Manukau East Ross Robertson was undecided on his vote, but said changing marriage laws had stirred "disquiet" in his electorate, which he described as the "spiritual belt" of New Zealand.


Asked whether Labour's association with the bill could hurt its vote at the next election, he said: "It has the potential to do that."

Ms Wall, who represented the Manurewa electorate, said she felt much of the opposition was based on misunderstandings of the legislation - one Pacific pastor had told her he "would rather go to jail than marry two people of the same sex".

She emphasised that churches would not be forced to marry same-sex couples.

The ructions over the bill will put Labour leader David Shearer under pressure to make sure the issue does not cause damage or split the caucus. Mr Sio's timing meant the issue overshadowed a major speech by Mr Shearer yesterday aimed at winning support from rural and provincial New Zealand.

Mr Shearer said yesterday MPs should not be "assassinated" for their voting choice, because a conscience vote allowed people to vote according to their beliefs or the will of their constituency.

Pacific Islands Advisory Group chair Uesifili UNasa did not think Labour's support would suffer because the public debate on marriage did not reflect the diverse range of views within both the church and Pacific Island communities.

Prime Minister John Key said he expected most parties would have MPs who were strongly in favour and against.

"I'm not terribly surprised you're seeing William Sio making those comments when the Samoan community is typically quite conservative and he is quite a conservative MP anyway."

He said views in National also ranged from conservatives who were opposed to the more liberal who were in favour. Although he would poll his electorate, he expected to support the bill, saying that as Prime Minister he had wider responsibilities than as an electorate MP.

"You don't have the luxury of standing back. I think my constituents at least would understand that."

A Herald poll found 26 out of 34 Labour MPs would support the marriage equality bill at least at the first reading. Six MPs were undecided.

West-Coast Tasman MP Damien O'Connor also intended to vote against the bill. This was despite a local group of supporters of same-sex marriage taking him out for dinner on Tuesday night to try to win him over.

- additional reporting Greymouth Star