An Australian politician's claims that gay marriage could affect Asian trade are being rubbished this side of the ditch.

New Zealand's trade with China - our biggest trading partner - has actually increased by about $3 billion annually since 2012, the last year before gay marriage was legalised in 2013.

But that did not stop Australian Agriculture Minister Barnaby Joyce saying that his country could be viewed as "decadent" in Asia if it allowed same-sex marriage.

Joyce has claimed that allowing same-sex marriage in Australia had the potential to damage negotiations and its trading relationship in Asia.

"I think that what we have to understand is that when we go there [Asia], there are judgments, whether you like it or not, that are made about us and they see in how we negotiate with them whether they see us as - whether they see us as decadent," he said.

Joyce, of the National Party of Australia, said "in some instances" gay marriage would be viewed as decedent.

New Zealand Labour MP Louisa Wall's Marriage Equality Bill passed its third reading 77 votes to 44 in Parliament in April 2013.

She could not recall similar trade fears being raised in New Zealand prior to the passing of the bill.

Trade Minister Tim Groser, who voted in favour of the 2013 Marriage Equality Bill, said he knew of no evidence that showed same-sex marriage had a negative impact on international trade.

Federated Farmers National President William Rolleston said there was no link between New Zealand allowing gay marriage and trade.

He did not recall the issue being raised when New Zealand was considering the Marriage Equality Bill.

"Quite frankly, I think it is a pretty silly thing to say."

The 12 months after the New Zealand change was probably our biggest trading year with China and a recent dip was to do with "milk powder coming off the boil", not gay marriage, Rolleston said.

"People try to use these sort of tactics to try to scare the public."

Statistics New Zealand figures show exports to China were worth almost $10 billion in 2014, compared to $6.9b in 2012.

In Japan, New Zealand's second biggest Asian trading partner, exports were worth $2.9b, compared to $3.2b in 2012.

In Korea, the third biggest Asian trading partner, exports were worth $1.8b in 2014, compared to $1.6b in 2012.

Victoria University Professor Siah Hwee Ang, from the School of Marketing and International Business, said New Zealand's boost in trade to China had nothing to do with gay marriage.

The main drivers were food scares in China meaning Chinese were looking off shore, wealthy Chinese and state owned enterprises looking to invest in New Zealand, and a free trade agreement.

New Zealand's seat on the United Nations Security Council made us "represent a power to which China wants to associate with", he said.

Joyce's comments followed Conservative cabinet minister Eric Abetz last week saying Australia should not legalise gay marriage because no Asian country had done so.

Australia's debate over same-sex marriage is expected to come to a head when Parliament resumes in August.