The ACTU was granted leave to intervene at a hearing in Sydney to consider a pilot union's bid to force Qantas to pay its New Zealand pilots the same wages as their Australian counterparts.
The ACTU has argued the case will have major ramifications for labour hire practice in Australia.
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The tribunal heard pilots working for Qantas's New Zealand-based subsidiary, Jetconnect, are paid significantly less than Australian Qantas pilots because they are not eligible for Australian International Pilots Association (AIPA) membership.
Positions previously held by Qantas pilots are being lost to Jetconnect pilots, counsel for AIPA said told the hearing.
At the same time, Jetconnect pilots were being subjected to "significantly inferior working conditions".
Despite being set up to undertake domestic flights within New Zealand, Jetconnect now operates 154 flights between Australia and New Zealand every week and is effectively an operating division of Qantas, the tribunal heard.
Its New Zealand pilots wear Qantas uniforms, have Qantas staff numbers, and fly Qantas aircraft with travel routes determined by Qantas.
The tribunal heard the case could potentially determine the future conduct of Australian employers and lead to an increase in the offshoring of many Australian jobs.
However, Qantas disagreed, saying such language was "inflammatory".
The airline referred to a 2007 Fair Work Australia hearing which formed the view pilots of Jetconnect were not to be AIPA members.
Qantas argued AIPA should not be entitled to come back three years later and make an application to represent Jetconnect pilots.
Although a wholly owned subsidiary of Qantas, it argued Jetconnect operated its own airline service based in New Zealand.
Speaking to reporters outside the court, the acting president of AIPA, David Backhouse, said Qantas had attempted to "symptomatically marginalise Australian workers".
"I think that's a problem for not only pilots but also for all workers in all industries," he said.
Jetconnect employees were paid up to 40 per cent less than Qantas pilots, had inferior employment conditions and and did not receive the same superannuation entitlements, he said.
"Where those workers are doing the same job as Australian workers and in actual fact replacing Australian workers, Australian work legislation should apply to them," he said.
The hearing continues.
