The Act was proposed as a private member's bill by MP Louisa Wall in May 2012, and was drawn from the ballot in July of that year. It passed its third reading in the House of Representatives on 17 April 2013, and became law when it received the Royal Assent two days later.
On 14 May 2012, openly lesbian Labour Party MP Louisa Wall (Manurewa) stated that she would introduce a private member's bill, the Marriage (Definition of Marriage) Amendment Bill, allowing adult couples of any gender to marry, including same-sex and different sex couples. The bill was submitted to the members' bill ballot on 30 May 2012. Openly gay Green Party MP Kevin Hague (list) also submitted a same-sex marriage bill, the Marriage (Equality) Amendment Bill, to the ballot on 24 June. Wall and Hague stated that they planned to work together in support of whichever bill came up first.
On 26 July 2012, a ballot draw of five members' bills saw Wall's Marriage (Definition of Marriage) Amendment Bill drawn and introduced. In its introductory form, the bill would insert a definition of "marriage" into Section 2 of the Marriage Act 1955 as "the union of 2 people regardless of their sex, sexual orientation, or gender identity"; it would also replace Schedule 2 – the list of prohibited degrees of marriage – with a new schedule using gender-neutral terms.
Prime Minister John Key had stated that he would support any same-sex marriage bill in its first reading initially, but after Louisa Wall's bill was drawn Key stated that he would support it through all stages. Opposition leader David Shearer also declared his support for Wall's bill and said that Labour Party MPs would be allowed to cast conscience votes. New Zealand First announced it would abstain, and that the legalisation of same-sex marriage should be put to a referendum. However, at the first reading, all New Zealand First MPs opposed the bill, largely because abstentions reduce the majority needed for a bill to pass.
On 29 August 2012, the bill passed its first reading on a conscience vote with 80 votes in favour, 40 votes against, and one abstention. The vote was originally recorded as 78 votes in favour and three abstentions, due to the National Party whips forgetting to cast proxy votes in favour for John Banks and Peter Dunne (both National Party whips voted against the bill and therefore were in the wrong lobby to cast votes in favour).