The announcement was made in response to plunging coal prices that hit the profitability of the mine and other coal mines in Queensland and New South Wales.
CFMEU district president Stephen Smyth reportedly told the Daily Mercury that the union was concerned that the criteria for selecting the retrenched had seemed to change.
"Length of service at the mine, absenteeism, training skills and discipline was the criteria but the company weren't applying it how they should have been," Smyth reportedly said.
"They were starting to change the criteria.
"If people are to be laid off then ... everyone (should be) treated the same and not ... left up to management just because they may have a personal dislike or due to union involvement.
"We want it open and transparent, which in our view hasn't been occurring."
The CFMEU was in discussions with Thiess and held meetings with Fair Work Australia to resolve the issue, Smyth said.
"We're still working out the detail,” he said.
"Obviously Xstrata isn't squeaky clean but because Thiess is the employer they will be meeting with representatives of the union."
A spokesperson for Thiess told the Daily Mercury the dismissal process had been transparent.
"In order to give our Collinsville team as much notice as possible, we commenced the consultation process early in November 2012 – two months prior to confirmation of final production plans and redundancy numbers," the spokesperson reportedly said.
"We have provided a transparent process through employee consultation and regular discussions with union representatives which was fully implemented in line with the employee agreement to which the unions are a party.”
Collinsville’s eight-pit complex 80km southwest of Bowen hit a stumbling block earlier this year when 27 employees checked into hospital following gas-related complaints.
Coal in the 2750 hectare mine’s eight seams had been determined to be prone to spontaneous combustion due to intrusions and minerals such as iron pyrites found in the surrounding geology.
The mine produces 4.5 million tonnes per annum of thermal and coking coal.