Prime Minister Tony Abbott says his royal commission into union corruption will not conflict with existing criminal investigations or cases.
Mr Abbott announced the royal commission on Monday after a series of recent media reports about union bribes, kickbacks and links to organised crime.
He says the commission's work will not conflict with current cases before police or the courts.
"There's no reason that this royal commission should interfere with any of those," Mr Abbott told ABC radio on Tuesday.
He also defended setting up two royal commissions, including another into the former Labor government's botched home insulation program.
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"It's important to learn the lessons of past mistakes," Mr Abbott said.
"Where you've got systematic wrongdoing, a royal commission is the best way to expose it and change the culture."
Opposition Leader Bill Shorten again dismissed the commission as a political witch-hunt that would take years and cost millions.
"There is already an Australian Crime Commission in existence which has the same powers," he told ABC radio.
Nonetheless, he said everyone should co-operate with the commission and stressed Labor had no interest in seeing crooks manipulate unions.
The Construction, Forestry, Mining and Energy Union's Dave Noonan said the commission's terms of reference seemed to imply that corruption among employers does not occur.
"This is a not about dealing with corruption," Mr Noonan told ABC radio.
"This is a political exercise by Mr Abbott to weaken trade unions and throw mud at the Labor Party."
But Employment Minister Eric Abetz said others in the union movement had called for a royal commissions.
"There has been this overwhelming call for a full royal commission from all sectors of the community, including many union officials who have seen first hand what is going on," he told the ABC.
"They are disgusted by what they have seen."
Senator Abetz said the commission was an attack on corruption - both of unions and employers - not on workers.
