New Zealand lawmakers will hold a vote on allowing gay marriage after a proposal to change the law was listed on parliament's agenda on Thursday.

The plan, which was drawn at random from a ballot of proposals submitted by lawmakers, would alter the 1955 Marriage Act to say marriage is a union of two people regardless of their sex, sexual orientation or gender identity.

The exact timing of the vote has not been determined but it is likely to be within the next two months.

Parliamentarians are expected to get a conscience vote on the issue, meaning they will not be bound by party policy.

Prime minister John Key said in May he was not opposed to gay marriage but it was not a priority issue for his centre-right government.

New Zealand currently allows same-sex civil unions which enjoy the same rights and obligations as a marriage involving heterosexual couples.

However the lawmaker behind the proposed change, Louisa Wall - a member of the centre-left Labour Party who is openly gay - said the institution of marriage needed to be updated to be more inclusive.

"I hope that it becomes about what marriage means in modern society," she said.

"So for me, it's about if two people love each other and marry and commit to each other for the rest of their lives, it should be something we all celebrate."

AFP