Prime Minister John Key has hinted he will support a bill legalising gay marriage all the way through Parliament.

Debate on the issue of marriage equality was reignited last week when Labour MP Louisa Wall's member's bill was pulled from the ballot.

It would give same sex couples the same rights to marry as heterosexual pairs.

Key initially said he would support the bill to first reading but was undecided after that.

It is likely to be a conscience vote meaning MPs can make up their own mind rather than having to tow the party line.

But this morning Key told RadioLive there was no point voting for something in the first reading if you weren't planning on supporting it.

"If you're very unlikely to vote for it don't vote for it in the first reading."

He agreed that meant he was likely to give the bill his continued support.

"My view has been that if two gay people want to get married I can't see why it would undermine my marriage with Bronagh. I just don't see how that logic applies."

However, MPs could change their mind he said.

Key changed his vote on the prostitution law reform because of the "merit of the argument".

It was healthy that New Zealand was having the debate but it was unlikely to be as heated as homosexual law reform in the 1970s, he said.

He expected public opinion was "mildly more" accepting than opposed.

There would be plenty of people who were opposed to gay marriage, including the deeply religious members of the National Party caucus, Key said.

Lobbying campaigns would be waged by both sides, as they always were on such issues, he said.

- © Fairfax NZ News