An ACT government spokeswoman said the decision to abandon litigation had partially been made as the statutory timeframe, which says a case must be commenced within two years, had expired.

Canberra unions say the failure is further proof WorkSafe ACT had been compromised by its merger into Access Canberra.

The half-built Gungahlin Drive Extension bridge over the Barton Highway collapsed in 2010 when more 400 cubic metres of concrete - weighing more than 800 tonnes - was being poured onto the span on August 14.

Fifteen workers were injured and nine hospitalised.

An engineers' report, commissioned by the ACT government and published in the weeks after the accident, found a litany of structural and planning failures led to the accident.

In the aftermath, a bridge engineering expert with 25 years experience, said bridge collapses like that on Barton Highway happened in third world and developing nations, but rarely in Australia.

WorkSafe launched its own investigation, and, despite the damning findings by other reports, seven years later has confirmed charges will not be laid

"Following advice from the Director of Public Prosecutions in 2016 this matter was not progressed to prosecution," the spokeswoman said.

"This, in part, was due to the length of time taken on the investigation by WorkSafe ACT which would have inhibited prosecuting this matter successfully.

"The DPP also considered that the current statutory timeframe of two years should inform decisions to undertake prosecutions as it was a reflection of the position of the Legislative Assembly.".

WorkSafe said a number of factors had contributed to the timeliness of the investigation, including a number of unrelated fatal accidents which occurred during the course of the investigation, the complex and technical nature of the matter, and the timing on gaining expert engineering opinion.

The spokeswoman said the head of Access Canberra, David Peffer, provided assurances to the government that Worksafe ACT had undergone a series of policy, structural and governance changes which had "improved the agency's ability to response to such incidents and to ensure that prosecutions, where appropriate, can be made for unsafe work practices".

"Access Canberra understands that there may be some civil matters in response to this incident; therefore it is not appropriate to comment further at this time."

The announcement drew condemnation from Construction, Forestry, Mining and Energy Union ACT secretary Dean Hall, who described the failure to prosecute the accident as "shocking, disgraceful and sadly indicative of a broken and ineffectual safety regulator".

"Shame on the government and Worksafe, there's blood on their hands," Mr Hall said.

"That is the most disrespectful, appalling and weak-kneed set of excuses I have ever heard.

"The families of killed workers and those workers who's lives have be change horribly as a result safety failures must be sickened, angered, bewildered and disgusted, I am."

Mr Hall said the government needed to set up an independent work safety authority with the adequate expertise and skills in order regain the faith of workers and the community.

"Even in the face of the ongoing debacle of failure after failure of investigation, why can't they learn from there mistakes?

"The Getting Them Home Safely report four years ago called for the registration of engineers, what happened? Nothing."

Unions ACT secretary Alex White said the decision had been further evidence that the work safety regulator has been severely weakened by its merger into Access Canberra.