The gay marriage bill has passed its first reading in parliament by 78 votes to 40.
The strong majority means it will almost certainly get safely through its remaining stages and become law, even if some MPs change their minds.
The first reading debate was dominated by MPs supporting the bill.
Of the 10 who spoke, only three opposed it.
Labour's Louisa Wall, who drafted the bill, was congratulated by her colleagues after the vote and the packed public gallery erupted into applause.
During the debate the bill's supporters argued that current law is discriminatory, denying New Zealanders equality.
Those who spoke against it referred to opposition from churches and said marriage was a sacred act between a man and a woman.
The bill has been sent to a select committee for public submissions.