The allegations come as his former union colleague and factional ally Victorian Labor MP Cesar Melhem, faces fresh allegations he used an industry-bankrolled slush fund to buy his way to political power. The fund, known as "Industry 2020",  was first revealed by Fairfax Media in December, 2012.

During a day of testimony  potentially troubling to Mr Shorten, the royal commission probed the Australian Workers Union's deal with Victorian mushroom grower Chiquita Mushrooms in the early 2000s, when Mr Shorten was union secretary.

Questions were raised about $4000-a-month payments the union received over six months in return for what the union claimed was for health and safety training.

However, former manager of the Mernda-based mushroom farm operator Stephen Little said he could only recall one training session that lasted "an hour and a half" or two hours at most.

Mr Little also acknowledged that, at least in part, the payments were intended to buy industrial peace.

"It's my understanding that at the time if there wasn't some movement on our part at least, it could lead to, let's put it, a lack of harmony within the workforce," he said.

Further evidence on the Chiquita deal will be heard at the royal commission's hearing in Melbourne on Tuesday.

The AWU was also probed on Monday over claims of murky back-room deals relating to slush fund Industry 2020, which was controlled by former secretary Mr Melhem and generated up to $500,000 for his factional power plays.

Financial records of the fund, tabled at the Melbourne hearing, revealed a string of cash payments made out to Labor Party factional powerbrokers and companies associated with factional allies, including:

- up to $100,000 to candidates seeking election for the disgraced Health Services Union;

- cash cheque payments to a suburban soccer club in Melbourne's west linked to Labor branch-stacking;

- cash cheque payments to a string of factional players including Labor Party Victorian secretary Noah Carroll, at the time secretary of Mr Melhem's Labor Unity faction; and

- tens of thousands of dollars left in Industry 2020 when Mr Melhem entered Parliament were transferred to other slush funds including one run by officials from the Shorten-aligned plumbers' union.

Mr Melhem, who had been Victorian secretary of the AWU before being parachuted into a safer upper house-state Parliament seat in 2013, was asked whether he had used the flush fund and union resources to buy his way to party preselection.

"You had, during the time you were state secretary, been able to use the resources of AWU to benefit Industry 2020 ... and you were able to advance your own position by ultimately obtaining the position that you hold today," counsel assisting the royal commission Jeremy Stoljar said.

Mr Melhem strongly rejected Mr Stoljar's accusation and claimed he had no interest in running for state Parliament until "a few weeks" before nomination.

But Mr Melhem said he regretted using cash cheques for some payments made from the fund.

"I thought I was doing the right thing but winding the clock back, it probably is something I should not have done," he said.

The commission also heard that powerful employers were promised a "good working relationship" if they bought expensive fundraiser tickets that helped generate cash for Mr Melhem's slush fund.

Emails revealed discussions with employers in which the AWU assured them that attending a $550-a-head lunchtime function at Flemington Racecourse in 2008, to hear then deputy prime minister Julia Gillard speak, would guarantee good relations with the union.

In the email, Visy Recycling operations manager Leigh Stewart said the company had been notified of the fundraising event by an AWU organiser, "who reminded me of how Visy's support leads to such a good working relationship with the AWU".

"Visy has supported AWU events like this in the past but for mine there is little or no value in this one [other] than political," the email said.

Mr Shorten's office said it had not responded to specific claims from the commission, "and would not start today".