Prime Minister John Key says he will vote for a bill to legalise gay marriage through all its stages if it passes its first reading.
The private member's bill in the name of Labour MP Louisa Wall has been drawn from the ballot at Parliament, and when it comes before the House a conscience vote is expected.
Mr Key says in the past, as an Opposition MP, he voted for civil unions and prostitution law reform at their first reading but then voted against them later on the basis of opposition from his constituents.
He says as Prime Minister he takes a different approach.
"When you become Prime Minister you're just not in the position to really have that sort of luxury, and in the end you're forced to make a decision pretty much early on.
"Ultimately, you don't have the luxury of sitting back so much."
Mr Key has already said he will support the bill through its first vote.
New Zealand First leader Winston Peters is calling for a binding referendum on whether gay marriage should be made legal.