Josh Cullinan, a former cleaner, exposed the scandalous wage deal the "shoppies" union did with the Coles supermarket chain, which ripped off tens of thousands of workers. And he did it in his spare time.

Woolworths, Australia's largest employer, has an almost identical agreement to Coles, meaning many of its workers have undeniably been underpaid. McDonald's workers are even worse off. It is likely hundreds of thousands of workers have been underpaid.

The fall-out for the Shop, Distributive and Allied Employees Association (SDA) could be immense.

Yet Cullinan should have been a great potential hire for the country's biggest right-wing union. Raised a Catholic, the young Cullinan spent time cleaning the meat rooms at Woolworths and working at petrol stations.

He had even spent a year in a presbytery while working for a Catholic organisation helping young workers, often in retail and fast food. He appeared to tick a lot of boxes for a big role in the "shoppies".

But his 2002 job interview with them did not go well.

"I was told in my job interview that if I worked for them I'd have to join the ALP and I'd have to attend ALP conferences," Cullinan recalls. "And [at the conference] I was to vote against abortion, rights for homosexuals and a whole lot of conservative agendas."

So instead of starting a career at the "shoppies", Cullinan took a job at the pulp and paper division of the CFMEU and later the National Tertiary Education Union.

A sliding doors moment for all, perhaps.

This week the Fair Work Commission found an agreement between Coles and the SDA covering 77,000 workers, failed a crucial legal test that all workers must be better off than the award, the wages safety net.

Cullinan's work, in conjunction with Fairfax Media, has uncovered well over $150 million a year in underpayments at Australia's largest three employers. All involving the country's biggest private sector union, the Labor Party's largest affiliate.

In the wake of this week's events people are asking, "Who is this guy?" It is a good question.

Cullinan has never had a high profile as a union official.

But for years, in his own low-key way, the 40-year-old from Melbourne's outer suburbs has been a prolific activist for fast food workers, casual workers, academics and for Bangladeshi workers in sweatshops.

In 2010 he was the Labor candidate for the state seat of Ferntree Gully but failed to be elected as the Brumby government was defeated.

In person, he's direct, personable, highly intelligent and fearless. He's still a Labor member and has just won one of the most significant wage underpayment cases in Australian history. An ideal political candidate? Perhaps not.

Cullinan said he knew when he exposed the SDA and Coles there would likely be consequences. "I was well aware that if I had any hopes of a political career in the future that's going to be somewhat stymied."

This week the head of Australia's union movement, ACTU secretary Dave Oliver, lent strong support to the SDA over the deals with Coles and even the anti-union McDonald's, where, as Cullinan and Fairfax Media revealed, workers are paid up to a third less than the award.

Incredibly, the ACTU is campaigning this federal election to "protect" penalty rates. Yet it is backing a union that does deals that cuts them or removes them entirely with little compensation for huge numbers of workers.

Oliver's curious position highlights the power the SDA is able to wield in the union movement. It has had a similar influence in Labor, where it has undoubtedly slowed the progress towards same sex marriage.

Bill Shorten, belatedly, this week was forced to distance himself from the union, telling ABC Radio in Queensland: "I am not here to defend the SDA."

But these dirty wage deals should not be a surprise to anyone in the ACTU or ALP.

The SDA emerged from the 1950s as a fiercely anti-communist union backed by B A Santamaria's ultra conservative National Civic Council. It is now one of the last remnants of the Labor split and the era of Cold War politics.

It has morphed into an organisation with incredibly close relations with employers – it even pays them about $5 million a year in commissions, ostensibly in return for deducting its members fees from their payrolls.

In return, the union maintains a large membership, giving it great influence in Labor on social policy.

Cullinan politics were also influenced by Catholicism. But he was attracted to the Young Christian Workers (YCW) movement where he worked in the 1990s and early 2000s. "The history of the YCW was it was the progressive socialist left of Catholic social action," he says.

His dogged pursuit of the SDA has its roots from his time at the YCW working with retail and fast food workers. He knew something was amiss. Eventually a senior labour movement figure told him to have a close look at the SDA.

"I came to understand there was this great secret of the union movement."

Over the next decade, in his spare time, he helped workers at Baker's Delight and at 7-Eleven. He set up a blog that raised concerns about SDA deals with big retailers such as Target. It was the slow boring of hard boards. But he pressed on.

Last May, Fairfax Media ran a series of stories on the SDA's deals with retailers and its sway over Labor on social issues. The next day Cullinan got in touch about Coles, and later, Fairfax ran a prominent report exposing it.

Cullinan had obtained a Coles store roster from Melbourne's west and had then crunched the data to show that most workers were underpaid.

Things moved quickly after that. A case was launched at the Fair Work Commission with the backing of the meatworkers' union. There was an early big win where Coles had to concede 27,000 casual workers and thousands of young workers were being underpaid. It likely cost Coles $15 million a year.

But they wanted more. A Coles trolley operator, Duncan Hart, joined the Fair Work case, and helped by a gun pro bono lawyer Siobhan Kelly, they took the case further.

This week's ruling will require Coles to pay higher penalty rates, or adjust rosters, or have its agreement with the SDA scrapped. It could cost Coles an extra $70 million a year in wages. It's a watershed moment for Australia's low paid.

"On the back of the current case it is going to be difficult for the SDA to sustain its practices and business model," Cullinan says.

There might come a point where the big supermarkets, forced to pay award rates, see no reason to deal with the SDA, as they can no longer deliver cut-price wage deals.

This raises the possibility that there will be no reason for Coles and Woolworths to maintain their close ongoing relationship with the union. That, in turn, is a relationship that delivers the union's Catholic, socially conservative leadership enormous political power within Labor.

Couldn't that leave supermarket workers worse off?

Cullinan can't see how it could be worse, with workers now paid well under minimum legal rates. He is hopeful change will come.

"The broader union movement would be much better off with a progressive union representing these workers," he says, "even if [that union] would be much smaller."

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