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

Vows were exchanged in venues ranging from an airliner cruising at 30,000 feet (9,150 metres) to a historic bath house as gay men and women took advantage of the law change.

The Campaign For Marriage Equality said it ended an 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," spokesman Conrad Reyners said.

The amendment to the Marriage Act was passed by parliament in April but did not come into effect until Monday.

Two radio stations competed to host the first same-sex wedding, with the ceremonies broadcast live during their breakfast programmes.

In reality, the nuptials took place around the same time, at 8.30 am Monday (2030 GMT Sunday), after the government offices that issue marriage licences had opened.

Reverend Matt Tittle from Auckland's Unitarian Church married one of the couples, Tash Vitali, 37, and Mel Ray, 29.

"It's history in the making," he said. "Hopefully it will help other countries to do the same and help New Zealanders to realise that everyone has worth and dignity no matter who they love."

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 American gay rights campaigner and actor Jesse Tyler Ferguson 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, conservative lobby group Family First 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," national director Bob McCoskrie said.

The Anglican Church has also asked its ministers to refrain from conducting same-sex weddings pending a report to its general synod next year.

New Zealand decriminalised homosexuality in 1986 and has allowed same-sex civil unions since 2005.

The Department of Internal Affairs said the number of applications for marriage licences being downloaded from its website had been running at triple the normal rate in the lead up to the law change.

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

Among the first to be married were Australian couple Paul McCarthy and Trent Kandler from New South Wales, who beat 300 other pairs to win a Tourism New Zealand competition.

Their wedding will not be legally recognised at home but McCarthy said he hoped that day would come and the ceremony at Wellington's Te Papa museum showed "we don't have two horns, we're not freaks (and) that there's nothing to fear from marriage equality".

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.

Tourism New Zealand chief executive Kevin Bowler said the country would market itself as a same-sex destination.

"New Zealanders are incredibly tolerant of people with different lifestyles, so I'm very confident the industry will embrace this opportunity," he told TV3.

"I think a lot of marketers underestimate the value of same-sex couples."