Four years ago, her son Ben Catanzariti was killed when a faulty concrete pump boom collapsed on him at a construction site on the Kingston Foreshore.

The death sparked significant pressure on the government to get tough on work safety, and prompted a high-profile prosecution of a multinational concreting company and an engineer, who were responsible for the machine's maintenance.

On Monday, prosecutors were forced into an embarrassing backdown, dropping the charges against both defendants.

The decision means that, despite all the tough rhetoric since Mr Catanzariti's death, four years on noone has been held to account for the failures that caused the boom to collapse.

It has devastated Mrs Catanzariti, who said the justice system doesn't give working classes a chance against money and power.

She arrived at court on Monday with her son's ashes and wearing a jumper that rescuers tore from his body as they desperately tried to save his life.

"This is my son, I don't get that back," Mrs Catanzariti said.

"And I can't bring Ben back, I know that. But I guarantee there will be another death in the ACT because charges have been dropped."

Schwing Australia Pty Ltd and NSW engineer Phillip James O'Rourke were responsible for maintaining the concrete pump that collapsed on July 21, 2012, killing Mr Catanzariti and seriously injuring two others.

It had been alleged that the pump had only recently been serviced and that bolts were incorrectly tightened, and then not checked properly.

The case was beset by delay after delay, but reached a shocking finality when it came to court on Monday, where prosecutors said they had no evidence to offer.

A new expert report, commissioned by Schwing, had found the failure of the bolts was not due to incorrect tightening, but due to a "metallurgical phenomenon" called hydrogen embrittlement.

WorkSafe ACT got its own report, which came to the conclusion that the bolts failed due to stress corrosion cracking.

Both reports dealt significant blows to the prosecution's theory pinning responsibility to Schwing and Mr O'Rourke.

The CFMEU, who formed a guard of honour for Mrs Catanzariti as she walked into court, say the case raises serious questions about the abilities and expertise of WorkSafe ACT and the Director of Public Prosecutions in handling complex, expensive work safety cases, particularly when they are pitted against wealthy multinationals.

Mrs Catanzariti said she felt abused by the process, and failed by those who told her they were there to help.

"I now realise that Australia's legal system has little to offer ordinary people like us," she said in a written statement.

"Justice only exists for those cloaked in money and power."

CFMEU ACT secretary Dean Hall called for an investigation into the failed prosecution.

He said the case would make the ACT look weak on work safety, particularly in the eyes of the construction industry.

"This has been one of the most high-profile cases in the history of the ACT as an industrial death," Mr Hall said.

"To come out of that court and be in a situation where it's embarrassing that the DPP stood to her feet and said that they tender no evidence."

"It's an absolute disgrace, it's something as a society, as a community, we need to reflect upon."

The death is likely to return to the ACT Coroners Court for the continuation of an inquest.

Mrs Catanzariti and the CFMEU have called for that to be conducted swiftly.

WorkSafe ACT, who gathered the evidence against Schwing and the engineer, issued its own statement quickly after the court appearance on Monday.

Work Safety Commissioner Brett Phillips said the new evidence was unforeseen and that the DPP had now concluded it could not prove its case to the high standard required.

"The decision was considered carefully and not taken lightly," Mr Phillips said.

"Mr Catanzariti's death remains keenly felt and WorkSafe will continue to support the Catanzariti family in any way it can."

DPP Jon White issued a similar statement, outlining the reasons it was forced to drop the case against Schwing and Mr O'Rourke.

He said there were now no "reasonable prospects for conviction".

"The death of Mr Catanzariti remains keenly felt," he said.

"The decision to discontinue the prosecutions was considered carefully and was not taken lightly."