Gay marriage proponents gained further ground as conservative New Zealand Prime Minister John Key said on Monday that, using conscience vote, he would likely support a Labor-sponsored bill on same-sex union.

"My view has been that if two gay people want to get married then I can't see why it would undermine my marriage to Bronagh," Mr Key said in an interview with RadioLive NZ, highlighting his belief that gay marriage should not be viewed as a direct threat to the institution of marriage.

Mr Key stressed too that his support for the bill filed by Labor MP Louisa Wall would be sustained through its third and final reading, allaying fears that he would make a turnaround as in previous occasions.

He added that debates on the legislation must be allowed to proceed while for his part: "I haven't spent a lot of time thinking about it. I'm going to give myself a little bit of room."

The NZ prime minister hinted too that his position on the issue will not sit well with colleagues in the country's conservative government, many of those opposing his stand will come from the deeply religious block, Mr Key pointed out.

Mr Key's positive view on gay marriage came as recent polls showed that some 64 per cent of Kiwis support the bill, which the Australian Associated Press (AAP) said is also slated to be backed by NZ Greens and Labor.

The NZ government endorsement was made following the boost provided earlier this year by U.S. President Barack Obama and British Prime Minister David Cameron on similar proposals pending on their respective nations.

In a response, Ms Wall said she is more optimistic than ever that Mr Key's positive attitude on the issue will strengthen the chances of her bill eventually overcoming the debates at the parliament.

"At the end of the day (Mr Key's) vote and my vote are worth the same. But ... I don't under-estimate the Prime Minister coming out really clearly and signalling to the rest of the country that we shouldn't be afraid of having this discussion," The Otago Daily Times reported the Labor MP as saying on Monday.

The New Zealand development is in direct contrast to the situation in Australia, where Prime Minister Julia Gillard had expressed her clear opposition to gay marriage, which is parallel to the conservative Liberal-National Coalition led by Opposition Leader Tony Abbott.

But unlike Mr Abbott, who warned that Coalition members supporting the proposal will be penalised, Ms Gillard said she would allow Labor MPs to vote on gay marriage using their conscience in the event such a bill would reach the Parliament for debate.

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