The overwhelming support for a bill to legalise gay marriage has stunned Prime Minister John Key, but he says its passage through its first vote is exciting.

Mr Key is in the Cook Islands attending the Pacific Islands Forum and missed the bill's first vote in parliament on Wednesday night (NZT), but cast a proxy vote in favour of the legislation.

The bill passed its first reading by 80 votes to 40, with one abstention.

The strong majority means it will almost certainly get safely through its remaining stages and become law, even if some MPs change their minds.

Mr Key told media in Rarotonga the result was "a strong endorsement for the equality of rights".

"It was an overwhelming result and I think it puts the bill in a pretty strong place to potentially become law, given you've had two-thirds of parliament voting for it," he said.

However, it was no surprise the bill passed its first hurdle.

"I think New Zealand's always been a country that's really stood up for equality of rights and egalitarian society, being the first country to give women the vote. We've been making slow progress in a number of other areas."

Mr Key believes the debate will grow louder and get "quite ferocious" as the bill progresses.

"All I've said to my MPs is 'I think you should exercise your conscience as you see fit and do what you believe is right'."

He said the bill is "relatively straightforward" and doesn't see the need for any amendments.

"Essentially it is mirroring the rights that a heterosexual couple have."