Labour MP Louisa Wall has come under intense pressure in her own electorate, with a large group of churches issuing a strongly worded warning to drop her bill to legalise same-sex marriage.

Twenty-five pastors from South Auckland churches signed a letter to the MP for Manurewa saying they oppose her proposed change to the Marriage Act and want it withdrawn.

"Over the last decade the Labour Party has passed laws that have weakened the family unit and angered their traditional supporters. This proposed bill would be another blow," the letter said.

The South Auckland electorates of Manurewa, Mangere and Manukau East are heartland Labour territory, and the party's MP for Mangere, Su'a William Sio, has already said he will vote against the bill.


Green Party rainbow issues spokesman Kevin Hague said MPs were dealing with a deluge of emails and letters, some of them "toxic", which warned that supporters of the bill would face repercussions at the polling booth.

But he offered reassurance to fence-sitting MPs who feared a backlash in the 2014 election if they voted in favour of gay marriage.

Yesterday, he released an analysis of previous conscience votes and the following election results. It showed that the vast majority of electorate MPs who supported homosexual law reform, anti-discrimination law changes and the Civil Union Bill held their seats.

"Right across the three different votes you would conclude that there's clearly no evidence that opponents were able to alter electoral outcomes," Mr Hague said.

He added: "I would suggest that the person who is more likely to vote on this as a single issue would be the person who is benefiting from it."

Of the 42 electorate MPs who voted for homosexual law reform in 1986 and contested the next election, all but one were re-elected. Three opponents of the legislation lost their seats.

Nine electorate MPs who supported civil unions in 2004 lost their seats in the following year's general election, but Mr Hague argued that this was because of other factors such as a huge swing from Labour to National and the emergence of the Maori Party.

The records showed that no South Auckland MP had lost a seat after voting in favour of homosexual reforms. Ms Wall's predecessor, George Hawkins, voted for civil unions in 2004 and kept his seat in the 2005 general election.

Among Pacific Island politicians, National list MP Alfred Ngaro plans to vote against a law change while Labour's Charles Chauvel and Kris Faafoi support it.

National MP Peseta Sam Lotu-Iiga is undecided and New Zealand First MP Asenati Lole-Taylor is seeking a referendum on gay marriage.

Homosexual Law Reform (1986)
1 (out of 42): MPs who voted yes, lost electorate seat
3 (out of 32): MPs who voted no, lost electorate seat

Civil Union Bill (2004)
9 (out of 36): MPs who voted yes, lost electorate seat
3 (out of 28): MPs who voted no, lost electorate seat.