Crime will rise if gay couples are allowed to marry, says the head of the country's victim lobby group.

Sensible Sentencing Trust leader Garth McVicar has submitted to Parliament that changing the law to allow same-sex marriage will be yet another erosion of basic morals and values in society which have led to an escalation of child abuse, domestic violence, and an ever-increasing prison population.

"The marriage amendment bill will not benefit society at all and will ultimately have detremetal (sic) effect on crime at all levels," the submission read.

Labour MP Louisa Wall's Marriage (Definition of Marriage) Amendment Bill is currently before a select committee, which will report to Parliament on February 28. If passed, it could see gay marriage legal by May next year.

The committee has received over 20,000 submissions on the bill.

Green MP Kevin Hague, who sits on the select committee, said after reading the vast majority of submissions, McVicar's was the only one linking crime to gay marriage.

"Although it echoes a number of submissions that say marriage has been the same way for a long time and that if you tamper with it there will be lots of unforeseen circumstances," Hague said.

He said he had not read any evidence to suggest that allowing gay people to marry would create a dysfunctional society or increase crime. "I suspect that underlying this submission is a prejudice against gay people. If you break the argument down, it is manifestly nonsensical."

McVicar's statement said that he considered the bill would be a further erosion of what he considered to be essential basic values and morals that have "stood the test of time for centuries".

"Furthermore, the bill represents a further decay and erosion of the traditional family that society has been founded on."

He said while much good work has been done recently to reduce crime and ensure better treatment of victims, the bill had the ability to destroy that good work.

Criminologist Dr James Oleson, from Auckland University, an expert in deviance, said he was not familiar with any research that would suggest homosexuals would be responsible for a disproportionate amount of crime.

For McVicar's argument to be plausible, he said, it would have to be proved that allowing homosexuals to marry degraded morality, and that this produced more crime. "But I'm not sure that's true." The more common drivers of crime were economic issues, alcohol, drugs, education and employment and family dysfunction, Oleson said.

McVicar did not respond to requests for comment.