Businesses warn a new super-union combining textile workers, dock workers and the CFMEU will hold Australia's supply chains to ransom "from pit to port".
But the newly-christened CFMMEU - with the extra 'M' - says it will fight for workers to get pay rises and corporations to pay tax.
The Construction Forestry Mining and Energy Union, the Textile Clothing and Footwear Union of Australia, and the Maritime Union of Australia will become one union from March 27.
The Fair Work Commission on Tuesday approved the creation of the CFMMEU, which has added "Maritime" to the name and will have around 144,000 members.
"Big business has too much power, we have record levels of inequality in our community, and working families are finding it hard to make ends meet," secretary of the new union Michael O'Connor said on Tuesday.
But the Australian Mines and Metals Association says the CFMMEU will control the supply chain from mines to ports.
"It beggars belief that the intention of our workplace laws is to allow two unions with a history of law-breaking and many outstanding proceedings to merge," the association's Amanda Mansini said.
The Master Builders Association says it is hard to imagine how a merger "with ramifications so dire" would be allowed to happen.
"The key partners in this 'super-union' seem to take pride in taking illegal action, believing they are above the law which applies to the rest of the community," chief executive Denita Wawn said.
The two organisations failed to stop the merger in the Fair Work Commission, but they are considering their options to stop what they can "an impending disaster".
The new union's international president Paddy Crumlin said it will fight to defend the interests of Australian workers against corporate self-interest and attacks on workers' rights.
The federal government in 2017 warned the merger would create "industrial chaos", but legislation it introduced trying to apply a public interest test to mergers didn't get past the Senate.
Workplace Minister Craig Laundy said it was not unreasonable to ask if unions who regularly break the law and aren't deterred by fines should come together.
"There are limited grounds for the government to intervene in these matters, even when the merger can have a major economic impact," he said.
Opposition Leader Bill Shorten said if the government was so worried about unions it should look after workers better.
"Last financial year corporate profits were up 20 per cent, wages went up two per cent. We are seeing an unequal distribution of national income," he told reporters.
Australian Associated Press