Cockatoo Coal has cut 40 jobs from its Baralaba coal mine in Queensland’s Bowen Basin.
Miners were told of the job cuts when they were handed letters at the start of their shifts, The Morning Bulletin reported.
The CFMEU’s Stephen Smyth said workers at the mine were not represented by the union.
"If these miners have been notified by a letter then that is really unacceptable to drop a bombshell like that," Smyth said.
"I don't know the fine details of their contracts but unfortunately for them when there's an agreement in place between the employer and employee without the union, this type of thing happens."
Cockatoo Coal has not responded to Australian Mining’s request for comment.
The Baralaba complex is made up of Baralaba North, Baralaba Central, and Baralaba South.
On its website, the company said its focus is on the recently-opened Baralaba North mine.
First coal was mined at Baralaba North in May last year and the expansion was said to have created 200 jobs.
Following its opening the mine will ramp up to one million tonnes per annum, and then following approvals, increase production to 3.5 million tonne per annum.