Prime Minister John Key has said he's not against gay marriage, on the same day US President Barack Obama announced his support for it.

In a statement by his press secretary today, Key said he is "not personally opposed to gay marriage.'' He previously declined to publicly state a position.

Obama became the first US president to publicly announce he is in favour of same-sex marriage today, in a high-stake intervention in a pre-election debate in American politics.

Key said the issue may come up again in New Zealand.


"It is possible Parliament may consider a members bill at some stage," Key said.

Labour leader David Shearer said he "fully" supported marriage equality in principle but would like to see the detail of any legislation before giving it formal support.

Labour's justice spokesman Charles Chauvel was more direct in his support for same-sex marriage.

"It was Labour policy in the last election to support the right of same-sex couples to marry."

Labour was looking at a private members bill to make it legal for same-sex couples to marry this term.

"I think the great thing about the President's announcement is it helps to highlight the issue of equality and keep it on the agenda and more and more New Zealanders are saying it's a no-brainer, people should have these rights," he said.

Maori Party co-leader Tariana Turia said she would support same-sex marriage on the basis that individuals and whanau had the right to choose for themselves whether or not to marry.

Greens co-leader Metiria Turei said she was pleased Mr Obama had taken a stand on what was a very difficult moral issue in the US.


"The US is very morally conservative on issues like sexual reproductive health, gay and lesbian rights and it's about time that the President took a strong stand in favour of the community and their right to be treated equally."

She said the Green party supported same-sex marriage in New Zealand and had argued for it when the Civil Union Bill was being progressed.

"The Civil Union bill was a negotiated agreement that everybody could live with," said Mrs Turei.

Their policy is that same legal rights and responsibilities should apply to all couples regardless of whether that couple is gay, lesbian, transgender or heterosexual, she said.

United Future leader Peter Dunne declined to comment, Act leader John Banks did not respond to questions, and Mana party leader Hone Harawira said he was undecided.

Mr Obama announced his change of position this week in an interview with ABC.

"I think same-sex couples should be able to get married," he said.

His announcement closed a process that had been years in the making and reflected changing views among Americans. Polls indicate a rising number of Americans - more than 39 per cent in a recent Reuters/Ipsos poll - favour allowing same-sex marriage.