Labor has launched a blistering attack on the unions royal commission ahead of Bill Shorten's appearance next week.
Mr Shorten, a former Victorian and national secretary of the Australian Workers Union, will attend a commission hearing in Sydney on July 8.
The commission is expected to quiz the federal opposition leader on what he knew about money received by the AWU from companies to pay for the union dues of workers and workplace training which may not have been delivered.
Opposition workplace spokesman Brendan O'Connor said that while Mr Shorten was willing to co-operate, the commission had been flawed from the start.
"Any independent observer would conclude to date that this commission has been biased, prejudicial and it has acted in a sensational manner about matters that are not proven," he told ABC television on Thursday.
"There are denials of natural justice."
The fact that Mr Shorten was the third Labor leader - following Julia Gillard and Kevin Rudd - to appear before the commission proved it was a "show trial".
Labor's view was that if there is evidence of misconduct it should be investigated by the Australian Crime Commission or police, he said.
In April, commissioner Justice Dyson Heydon defended the inquiry saying it was up to witnesses to make it their "personal duty to search their recollections with diligence, summon up their powers of careful and accurate expression and adhere unswervingly to proper standards of testimonial honesty".
The royal commission will follow up its Sydney hearing with three weeks of hearings in Canberra starting on July 13, where it will deal with issues surrounding the Construction Forestry Mining and Energy Union.
Its final report is due by the end of the year.
