Prosecutors have been forced to withdraw half of the charges against the Construction, Forestry, Maritime, Mining and Energy Union and secretary Jason O'Mara.

Revelations during eleventh-hour discussions between lawyers outside court on Thursday left prosecutors with little choice but to reconsider the future of the case.

Both the union and Mr O'Mara have denied allegations that during meetings with Canberra steelfixers and scaffolders to negotiate new employee agreements they pressed the businesses to collude on a fixed minimum price for their jobs in order to afford worker pay rises.

It's alleged the union threatened to run out of town any businesses that failed to fall in line.

On Wednesday, prosecutors withdrew the charges as they related to the steelfixers.

The union and Mr O'Mara have been fighting the case's committal to a higher court for trial.

The hearing in the ACT Magistrates Court this week has been frequently interrupted by the rolling disclosures of documents previously held back from the parties.

But on Thursday, revelations made by the defence to prosecutors in discussions outside court called into question the future of the entire case.

Prosecutor Rowena Orr QC told the court the new information had the potential to affect the course of the prosecution.

Further fact finding enquiries would need to happen and so a decision could not be made by Friday, she said.

The information was not something that the CDPP or the barristers could have found, she told the court.

Ms Orr said the material as a matter of fairness to the accused had been segregated from prosecutors.

Resolving the matter would be complex, she said.

Fact finders would have to communicate with the now non-existent Royal Commission into Trade Union Governance and Corruption, which ended in 2015.

There were also consideration being given to briefing independent counsel separate from the prosecution team to consider the matter.

An "ambitious but achievable" time frame to return to court would be four weeks, the barrister said.

Neil Clelland QC, for Mr O'Mara, highlighted that there was a "sharp distinction" between the knowledge of prosecutors and of the Australian Competition and Consumer Commission, which investigated and laid the charges.

He did not oppose time for the other side to consider the matter but urged the court to continue hearing evidence on Friday, given the potential for the case to stretch out to 2022 if prosecutors decided to press on.

And Rachel Doyle SC, for the union agreed, saying they could not risk wasting booked court time.

Mr O'Mara and the union are charged with two counts each of attempting to induce the businesses into cartel conduct.

Mr O'Mara could face up to 10 years in prison if found guilty while the union could face a $10 million fine.

The case returns to court on Friday.

Editors note: This story was updated on March 1, 2021, to reflect that prosecutors withdrew charges against the union and Mr O'Mara.

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