Dozens of same-sex couples have said "I do" after New Zealand became the first Asia-Pacific country, and only the 14th in the world, to legalise same-sex marriage.

Vows were exchanged on Monday, in venues ranging from an airliner cruising at 30,000 feet to a historic bath house as gay men and women took advantage of the law change.

The Campaign For Marriage Equality said it marked the end of a historical injustice and meant the love of all people was recognised as equal in the eyes of the law.

"A massive congratulations to the happy couples tying the knot today. Marriage equality has finally arrived in New Zealand," Conrad Reyners, spokesman, said.

The amendment to the Marriage Act was passed by parliament in April but did not come into effect until Monday, after the government offices that issue marriage licences opened.

Air New Zealand staged a special flight from Auckland to Queenstown where Lynley Bendall and Ally Wanikau tied the knot in the air after 14 years together, with Jesse Tyler Ferguson, American gay rights campaigner and actor from the hit comedy Modern Family, in attendance.

"To be married at 30,000 feet beneath strings of fairy lights with our children, friends and family as witnesses makes an already memorable day that much more special," Bendall said.

"It was surreal to have Jesse play a part in the ceremony too."

However, Family First, the conservative lobby group, said changing the Marriage Act was "an arrogant act of cultural vandalism" that politicians had pushed through without a public mandate.

"Social engineers including politicians and activists are expecting marriage supporters to drop their deeply held convictions because of the misguided decisions of politicians," Bob McCoskrie, national director, said.

New Zealand decriminalised homosexuality in 1986 and has allowed same-sex civil unions since 2005. Parliament passed a bill earlier this year that legalised same-sex marriage.

At least 31 same-sex couples had indicated they would marry on Monday, usually the least popular wedding day, while inquiries about staging same-sex weddings in New Zealand had been received from Russia, the US, Hong Kong, Britain, Singapore, Malaysia, Guyana and Belgium.

About 1,000 same-sex couples in Australia have indicated they plan to travel to New Zealand to marry, according to the Australian Marriage Equality lobby group.