The Shop, Distributive and Allied Employees Association's then national secretary Joe de Bruyn and the ACTU's then secretary Jeff Lawrence in 2011. Credit:Arsineh Houspian The father of four answers questions about pay and working conditions by highlighting their importance to family – husband, wife and children, that is. "Marriage is between a man and a woman; always was, always will be. It is based on what is innate in human nature." Politicians, marriage equality lobbyists and academics agree the SDA has been, and continues to be, a major obstacle to same-sex marriage reform in Australia. This week de Bruyn, 65, again weighed in, railing against deputy Labor leader Tanya Plibersek's call for a binding vote for Labor MPs on the issue when it returns to Parliament later this year. This, despite polls showing that almost three-quarters of the wider population supports marriage equality, a figure likely to be higher still among the SDA's own members – young, often female, department store, supermarket and fast-food staff, hairdressers and models. As other once great unions decline in size and influence, the SDA maintains a 200,000-plus membership, giving it proportionately more political clout in the ALP than at any time in its history.

Joe de Bruyn. Credit:Jesse Marlow In an era of union decline, and fast-changing social attitudes, how does it maintain its numbers, its political influence, and its resolutely conservative social agenda? Could it be that the tide of social change is washing the ground from under the SDA at the zenith of its numerical strength? Like the Dutch boy and the dyke, might de Bruyn be holding back the inevitable? The Catholic connection. De Bruyn cannot recall the last time he was on a picket line. Forthright and animated on matters of social morality, he sits back subdued when asked about industrial action.

In an expansive interview with Fairfax Media, ahead of his retirement as union secretary– he is now national president – de Bruyn recalls a picket over the closure of the failing Waltons department store, maybe in the 1980s. Even then, it is unclear if he actually joined it. He puts the rarity of industrial action by the SDA down to the fact that retail workers lack industrial muscle and leverage, and are disinclined to militancy. Few who know him doubt that de Bruyn has battled hard on behalf of members. Friends and foe alike point to the SDA's forthright opposition to the Howard government's WorkChoices. But critics – and they are many – have long contrasted the SDA's doctrinaire social positions with its moderate industrial approach. The shoppies are the last surviving of the Catholic-controlled "grouper" unions – as they were known in the 1940s and 50s – named after the industrial groups organised to counter communists in the unions.

Four unions left the Victorian ALP amid the seismic Labor split of the mid-1950s, and returned in the mid-1980s as part of Labor leader Bob Hawke's bid to shore up right-wing numbers in the Victorian party. The clerks, carpenters and ironworkers have since been taken over by rivals or subsumed by other unions. In recent weeks the shoppies have had an uncomfortable reminder of old factional battles with its own Melbourne offices picketed by the rival meatworkers union. The left-wing "meaties" are angry over the SDA's proposed national agreement with Coles which, they say, undercuts wages and conditions for many supermarket meatworkers. So too are they unhappy about SDA's recent template agreement with South Australian business, allowing penalty rates to be cut in exchange for higher wages. "The SDA is a tame cat union," says Paul Conway, secretary of left-wing Victorian meatworkers union. "Its primary interest is not its members but numbers in the ACTU and ALP, getting its people into Parliament, having an impact on issues like same-sex marriage."

De Bruyn defends his industrial record, and claims a tenfold increase in the award rate for shop assistants and, pay increases above inflation, since the mid-1970s. There have been recent wins on lifting wages for young workers. It is true that retail workers in Australia are some of the best paid in the world, but how much of this is due to the SDA, or Australia's relatively generous award system, is hard to untangle. Basic weekly adult rates are about $750 a week at Target and Woolworths. The award – the bare-bones rate set by the Fair Work Commission – is a touch more than $700 a week. In an era where private sector unions have dwindled elsewhere, it is notable that the SDA has maintained such a large membership and, therefore, the biggest power bloc in the ALP. Its strength is drawn, at least in part it seems, from its unusual closeness to major employers. The SDA provides big employers predictable wage increases, zero industrial disruption, and a co-operative single voice speaking for a large and dispersed workforce.

And, importantly, employers get to deal with the reliable SDA rather than, for instance, with its more restless rival, the National Union of Workers. The relationship was cemented with a landmark deal in the early 1970s whereby six big retailers literally signed up union members. Under the deal, membership exploded from about 57,000 to an eventual high of almost one quarter of a million. While strict closed-shop arrangements are no longer allowed in Australia, SDA organisers are also welcome at, and regularly attend, employee inductions. As Fairfax Media reveals on Saturday, the SDA pays as much as $5 million a year in commissions to big employers, including Coles and Woolworths, in return for payroll deduction of union dues. In a memo leaked to Fairfax Media, a Coles human resources manager refers to the company's "solid relationship" with the SDA but directs store managers to remove material from noticeboards by the troublesome meatworkers, "quietly, so it doesn't stir them up".

De Bruyn acknowledges "good relations" with employers like Coles and Woolworths, and a record of minimal disruptive action. "If we have a fundamental problem with a company I will go to the top of the company and say 'you have a problem and you have to fix it'." While such industrial coziness riles many fellow unionists, it has drawn praise from unlikely places. "Joe de Bruyn is a role model of trade union officialdom," says federal Employment Minister Eric Abetz​. "He is the type of official that gives trade unionism a good name." SDA critics were not surprised, therefore, when the Abbott government's brief for its royal commission on trade union corruption excluded Labor's largest union, instead targeting Bill Shorten's AWU and the militant CFMEU, among others. The omission is notable given the royal commission focus on slush funds and the use of resources for Labor politicking. The SDA is renowned as an active player in union-ALP factional politics. De Bruyn is a long-term member of the ALP national executive but says he leaves factional power plays to state branches.

Like other unions targeted by the commission, the SDA uses slush funds, including the mysterious Victorian-based Friends of Democracy, to finance factional battles including elections in the disgraced Health Services Union. In his 2004 "diaries" former Labor leader Mark Latham specifically writes about the shoppies' role in Labor's factional infighting, claiming "they practise the worst of machine politics". So, if the SDA delivers only modest wage rises for its members, and is preoccupied with Labor faction-fighting, why is the decades-old Catholic leadership not challenged and defeated by frustrated members? In part the lack of change is explained by the fact that the dispersed, casual and transient SDA membership makes organisation of a realistic challenge difficult. Latham may also have had part of the answer also when he described the shoppies as "fanatical and authoritarian".

De Bruyn never faced a contest in his 36 years as national secretary. While he retired from the position in 2014, he is now an active national president, a move common for ageing SDA officials which helps maintain continuity and control. Victorian secretary Michael Donovan began working for the union in 1977, and became state leader in 1996. In a 2010 election he received a Soviet-like 90 per cent of votes cast. Most SDA members will be surprised to discover that the union's constitution commits it to a paternalistic "safeguarding" of their interests, not only industrially but "morally", "socially" and "intellectually" as well. No need, therefore, to involve members in debates on matters such as marriage equality. Paid union staff are on a tight leash. The first clause of an organiser's employment contract from the mid-2000s, obtained by Fairfax Media, demands "loyalty" to the elected secretary "at all time". Clause two is headed "confidentiality". Such control and discipline by long-serving officials with shared conservative values at national and state levels, make the SDA leadership a formidable political force.

Labor number crunchers estimate the Victorian SDA and associated branch activists command about 20 per cent of votes on the floor of the key decision-making forum, the state conference. When appointed, the cabinet of the Andrews Labor government in Victoria included four SDA-linked MPs, including Deputy P James Merlino. In South Australia SDA influence is greater still, with the union the dominant force in Labor's powerful right-wing faction. Former Labor senator Linda Kirk paid the price of snubbing her SDA backers when in 2006 she supported the conscience-vote bill to permit therapeutic cloning of stem cells. She was dumped from the top spot on the Labor senate ticket and replaced by the SDA's own long-time president, Don Farrell. Last year WA senator Louise Pratt, who is openly gay, also lost her top senate spot and, consequently, her seat in Parliament, after being dropped in favour of 60-year-old SDA secretary, Joe Bullock.

The day before the WA senate election rerun last year, it was revealed Bullock had given a speech ridiculing Pratt's sexuality, and describing members of his own party as "mad". So too did the SDA play a key role in killing off the political aspirations of former Australian Workers Union national secretary Paul Howes, when it opposed his 2013 bid for the NSW senate position vacated by former foreign minister, Bob Carr. Howes attributed his loss in part to the SDA, and his support for same-sex marriage. "Maybe it would have been a different kettle of fish if I had a different view," he said at the time. Party insiders estimate that federally there are 10 to 12 MPs with some level of SDA link,including lower house members Tony Burke, Kate Ellis, David Feeney, Amanda Rishworth and Nick Champion. Senators include Helen Polley, Jacinta Collins, Bullock, Chris Ketter, Deborah O'Neill and Catryna Bilyk.

The union's power is made still greater through deft alliances and deals, a preparedness to "blow up" such deals, and an authority in de Bruyn's case that comes from being a respected and feared elder of the movement. Shoppies' support also crosses factions, with Julia Gillard, a left-wing recipient of de Bruyn's backing after she toppled then prime minister, Kevin Rudd, in mid-2010. The peculiar political coupling coincided with Gillard's surprise stance against same-sex marriage. In her memoir My Story she denies her admittedly "odd" position was about shoppie support, and insists it reflected her feminist discomfort with all marriage. Senior Labor insiders, including some from within her trusted inner circle, question this. They are in no doubt that SDA numbers were the real rationale. Yet Gillard also notes that attitudes in the community and ALP are at a "canter" on marriage equality, a fact that raises thorny questions for her party's most powerful affiliate, and the MPs it sponsors. Australian National University political scientist John Warhurst, says opposition to same-sex marriage "may well be a last-ditch moral stand for the shoppies' old guard".

"Sooner or later there could be generational change occurring on issues like this. I imagine the SDA leadership could come under increasing pressure from younger members and others in the broader right faction in favour of marriage equality and social change." Back in 2004 the ALP backed a Howard government amendment of the Marriage Act to define marriage as being between a man and a woman. By 2011 the party's position had moved dramatically, with the national conference writing support for marriage equality into the party platform. Not the first time on such issues however – stem cell research was another – the SDA fell back to backing a conscience vote, an outcome that allows it to continue pressing individual MPs to hold the SDA line. Rodney Croome, the national director of lobby Australian Marriage Equality, says that had Labor voted as a bloc, a same-sex marriage bill tabled in 2012 "would have had a fighting chance of passing both houses". In July 2014, research company Crosby Textor released a survey of 1000 Australians in which a massive 72 per cent supported marriage equality.

De Bruyn dismisses the figures but refuses to poll his members on the issue. He says he knows they agree with him "absolutely". "When we talk to our members about out these things they agree with us." Latham is scathing of de Bruyn's paternalism. A decade ago, as the stem cell debate raged, he accused the SDA of "peddling" religious propaganda "oblivious" to the interests of its members. "I could visit Woolies and Coles for the next thousand years and those young girls at the checkout would never talk to me about stem cell research," he says in his diaries. Some of the SDA's own MPs, notably frontbench South Australian Kate Ellis, have defied the union on marriage equality, and survived. Labor insiders also point out that many young ALP activists who now attach themselves to the SDA do so, not out of religious belief, but because the powerful shoppies are a good avenue to a political career. When de Bruyn joined the union fray in the Cold War climate of the 1970s unions were central players in the Australian economy and politics. Catholic activists like BA Santamaria, and younger crusaders including de Bruyn, saw the union movement as a key battlefield in the wars against both communism and social liberalism.

In the case of the former, the SDA was on the winner's side. But the Berlin Wall fell long ago. The morality war looks like a lost cause. With marriage equality a reality in 20 countries including Catholic-dominated Spain, Portugal, Argentina, and Mexico, Warhurst says Australia risks being internationally isolated on the issue. He says both major parties will be pressured by voters to "get with the modern world". That is not de Bruyn's world. He insists that "logic" will win the day and that Australians and their MPs will realise that same-sex marriage is wrong. If the law changes to allow it, he says, the law "is an ass". "The Pope will move on this issue before Joe does," says a left-wing federal Labor MP with an affectionate chuckle. And then what for "the shoppies"?