Fresh ructions have erupted in Labour as its conservative Pacific supporters question a gay marriage push and one-time leadership rival David Cunliffe is targeted by a whispering campaign over his ambitions.

Both issues are likely to rear their head at Labour's caucus next week after they overshadowed leader David Shearer's launch of a "heartland" tour designed to take the party's message to rural New Zealand.

Mangere MP Su'a William Sio yesterday broke ranks over a member's bill promoted by fellow Labour MP Louisa Wall and warned it would cost Labour the next election.

Shane Wenzlick SU'A WILLIAM SIO: Says he will be voting against the gay marriage bill.

He told National Radio the issue "cuts deep into fundamental beliefs" and would divide the community, but he later went to ground.

A backlash against Labour on moral issues, including civil unions and smacking, was blamed for it nearly losing the election in 2005 after its Pacific base, which is traditionally Christian and deeply conservative, stayed home.

Labour's Pacific sector vice- president, Efu Koko, said he had spoken out against the bill when it was flagged with party leaders before it went into the members' bill ballot.

'We've got more pressing issues for our people than this. This is something we don't need at the moment, we want to win the next election."

Labour's Pacific wing is set to debate the issue next month.

But Sio appears to be a lone voice among Labour's Pacific caucus, with Mana MP Kris Faafoi and list MP Charles Chauvel signalling their support for the bill.

Faafoi said he supported the bill on the grounds that all people should be treated equally, regardless of race, religion and gender. He said he had so far received only one letter taking issue with his stand.

Shearer said it was not a Labour bill and MPs from all parties would vote according to their conscience. "It's not something we put forward as the main thrust of where we want to go. We're about the economy and jobs and education. It's out there though and every MP has a right to be able to put bills in the ballot."

But the debate is one more headache for Shearer amid suggestions senior Labour MPs have been white-anting Cunliffe, his economic development spokesman, apparently in response to renewed speculation about his leadership ambitions.

One report suggested MPs thought Cunliffe was lazy and Shearer was disappointed in him.

But Shearer yesterday hit out at the speculation and said Cunliffe had his full backing.

There was no suggestion that Cunliffe would be demoted.

"God no, the guy's got a huge brain. He's really across economy policy. Hell no, I want him to be there and want him to keep doing what he's doing . . . I don't think there would be anybody else in the caucus who could do it as well as him, to be honest."