Mining and building industry groups have lodged an appeal against the creation of a super union.
The Australian Mines and Minerals Association and Master Builders Australia lodged their appeal with the full bench of the Fair Work Commission on Thursday and want a guarantee the merger won't go ahead until their case is heard.
But the unions say they've got the backing of members and will defend their right to merge.
The commission this week approved the amalgamation of the Construction, Forestry Mining and Energy Union, Textile Clothing and Footwear Union of Australia and the Maritime Union of Australia, to create the new CFMMEU.
Both industry groups previously tried to block the amalgamation, but it's due to take effect on March 27.
It's understood they're arguing technical grounds for their appeal.
The MUA says the unions will vigorously oppose the appeal and defend the rights of workers to engage in freedom of association.
"Our members have overwhelmingly supported this amalgamation and it should be up to them to decide whether they merge," national secretary Paddy Crumlin told AAP.
The Turnbull government wants parliament to pass laws subjecting union mergers to a public-interest test, but has yet to secure Senate crossbench support.
Tasmanian Liberal senator and former employment minister Eric Abetz described the merged body as an "unholy alliance of law-breaking union heavies".
Australian Associated Press