The mines and metals industry is warning of a monopoly over the supply chain if a super trade union is established.
The Construction, Forestry, Mining and Energy Union and the Maritime Union of the Australia have started official moves to merge.
"There is no doubt that the stability of the supply chain, from pit to port, is at risk if this merger is allowed to go ahead," the Australian Mines and Metals Association director of workplace relations Amanda Mansini said in a statement on Thursday.
The association is concerned the risk from merging unions will would translate to millions of dollars of foregone revenue for both state and federal governments.
It wants Employment Minister Michaelia Cash to urgently implement a public interest test on union mergers.
It would not be an anti-union law, Ms Mansini said.
"Unions that honestly and lawfully represent their members and always comply with Australian workplace laws should easily pass such a merger test."
Senator Cash has previously said the proposed CFMEU-MUA amalgamation would threaten employment growth and productivity.
© AAP 2017