Shop, Distributive and Allied Employees Association national secretary and Labor national executive member Joe de Bruyn said he had never heard of a group called SDA Members for Equality that challenged the union's policy in a submission to a Senate committee.

Mr de Bruyn said he believed the majority of the SDA's 212,000 members supported the traditional definition of marriage.

In the submission, SDA Members for Equality national convenor Duncan Hart said many retail workers supported changing the Marriage Act to allow same-sex weddings.

''The majority of these workers are young, and so are statistically more likely to support gay marriage equality,'' Mr Hart said.

''Despite repeated requests, the SDA has refused to survey its membership to ascertain their views on marriage equality. We know that many SDA members support the issue because we receive tremendous support from them.

''We also know many workers in the retail industry who have resigned from the SDA - or refused to join from the outset - because of the union's opposition to marriage equality.''

In a response to the submission, Mr de Bruyn said the union had ''some years ago'' discussed with a representative sampling of members the view that marriage was between a man and woman and received overwhelming support.

''We believe that this continues to be the view of the overwhelming majority of the members of the union,'' he said.

Mr de Bruyn also dismissed the suggestion that the union's voting structure meant that ordinary members and their views were not represented.

''Any analysis of the rules of the union at the national level and our various branches would make it clear that officers of the union and the decision-making committees at the branch level as well as the national council are elected directly by the members of the union on a regular basis as is required under the relevant parts of the Fair Work Act.''

The SDA has had a large number of Catholics among its leadership and has networks of supporters among state and federal Labor MPs.

A brief submission to the Senate inquiry by United Voice ACT union organiser and ACT Labor Assembly candidate Yvette Berry supported same-sex marriage. ''Our members should not be subject to any discrimination under federal law,'' Ms Berry said in the submission made on behalf of her union.

A submission from the ACT branch of the Australian Family Association argued that there was no political mandate for the Marriage Act to be changed.

The Canberra Atheist Church argued that the recognition of gay marriage was an important step towards providing legal and social equality for same-sex partners.