A group is calling for the Government to consider legalising multi-partner marriages.

The group set up a Facebook page just before the Marriage Amendment Bill passed through Parliament last week, legalising gay marriage.

A statement on the page described multi-partner - or polyamorous - marriage as "responsible, adult, committed non-monogamy," and said all committed loving relationships between adults regardless of number should be respected and given legal acknowledgement.

"Some Australian Greens have now got a lobby group going, there are several MPs around the world coming out as poly and poly-friendly and it seems the time is right to at least bring it to the attention of the New Zealand public and New Zealand parliament," the group said.

"This will be a long-term project but with the rest of the world getting on the bandwagon legal multiple partner marriages/unions may one day be accepted."

The "slippery slope" towards legalising polygamy, polyamory or incestuous relationships has often been an argument raised by opponents of the Marriage Amendment Bill.

Last year the member in charge of the bill, Labour Party MP Louisa Wall, and Conservative Party leader Colin Craig went head to head in a live online debate about marriage equality.

At the time Mr Craig predicted the bill would inevitably raise issues surrounding discrimination to people in polygamous or polyamorous relationships who could not marry and yesterday said it was "no surprise" the Support Legalise Polyamory in New Zealand group had started up and there was no doubt marriage would be further redefined in the future.

"We have opened the doors to this."

He said it had begun when the line was moved for civil unions.

"They said that was it, they'd never touch marriage again, and that's what they're saying now."

Mr Craig said legalising gay marriage created a precedent for further redefinition of marriage, which he had been against from the start.

Last year Ms Wall said marriage was based on love, which was why gay and lesbian people wanted the same rights.

"Marriage is the institution of two people who love each other committing themselves to a shared life together. This bill will not address polygamy or other forms of relationship."

Mr Craig said if people were intellectually honest they would see the arguments for legalising gay marriage could apply directly to those in other alternative relationships.

"If it's about love and commitment and respect, don't they have that? They would say they do."