Unions say the latest decision by the Fair Work Commission (FWC) involving Collie's Griffin Coal Mine workers will open the floodgates for multinationals and other companies wanting to slash wages and conditions.

On Thursday, the full bench of the Commission upheld an earlier decision which effectively cut the pay of maintenance workers by 43 per cent or $55,000 a year.

The Commission had previously approved an application by the Griffin Coal Mining Company to terminate the existing agreement and revert to award conditions, after a year of failed negotiations with the Australian Manufacturing Workers' Union (AMWU).

Griffin Coal argued that overly generous conditions were making it impossible for the company to turn a profit and expand.

AMWU state secretary Steve McCartney said all workers should be deeply concerned by the FWC decision.

"I think they've opened the gates, I think what they've done is sent a message to multinationals now if you want your wages and conditions lowered, then you come to FWC and we'll put them back on the award."

"Now we know Premier Coal, Muja and all those other places around that area will be looking for an excuse to get their money down, so they'll be using it to push down wages and conditions."

Mr McCartney said other companies were already making reference to the case in EBA negotiations.

"Everywhere we're negotiating at the moment, they put Commissioner Cloghan's decision next to them and they make sure that our delegates see it," he said.

"What they're trying to do is use that as a stick to drive down wages and conditions.

"If we have to get into the trenches to win this we will."

Unions WA secretary Meredith Hammat said the decision would have devastating consequences for all workers.

"This is a decision that gives significant unfair advantage to employers to be able to threaten or to actually be able to reduce take home pay for their workers by cancelling an agreement and we think it's a bad decision," she said.

"This is a matter that the Federal Government needs to look at, are we saying that it's acceptable for working people to experience cuts in their pay of up to 43 per cent because a company has sought to cancel an agreement?

"We think this shows the Commission is really out of touch with the consequences for working people and that concerns us greatly."

The pay cut for maintenance workers would not take effect until the FWC lifts a stay on its decision.

The FWC will reconvene on Monday to discuss the stay order.

Calls to Griffin Coal by the ABC were not returned.

The company has previously said it will continue to negotiate with workers on a new EBA.