The CFMEU says that Rio Tinto's redundancy package for its Blair Athol coal mine will discriminate against union members.
Earlier this year Rio announced that it will officially close the mine by the end of 2012.
It came after the miner pushed forward the date of the closure, slashed jobs at the nearby Clermont coal mine, and closes its offices in Sydney.
Blair Athol closes after nearly 30 years of production, with Clermont region general manager operations Dawid Pretorius explaining that the mine has been progressively scaling down since 2010.
"After close to three decades, Blair Athol Mine’s coal seams are largely mined out and the time has come to finish production," Pretorius said.
"Since 2005, Rio Tinto had planned to close the mine at the end of this year and consulted with its community consultative committee and workforce.
"However, as coal prices rose in recent times we looked to extend the life of the mine by mining a poorer quality coal and harder to reach seam for a few more years; unfortunately the recent significant drop in thermal coal prices and other factors such as rising costs and the foreign exchange rate mean this is no longer a feasible option and we will keep to the original plan to finish production in 2012."
Now mining unions are trying to finalise a new enterprise agreement for their members at the mine, as they believe under current conditions unionised workers will receive less than those on individual agreements, the ABC reports.
"Depending on how long you've been at the mine, that market allowance could be up to many … thousands of dollars," CFMEU spokesperson Glenn Power said.
"It's very different for every individual or particular member but certainly there's a substantial amount of money that our members are going to go without."
Unions workers will go to a vote next week to decide on whether to strike.
"Our members deserve equality … they do the same work, they should be treated the same," he said.
It comes as the two year rolling battle between the CFMEU and BMA comes to a close.