Although no one was injured, the prefabricated panel crushed a parked car and toilet block.

Construction, Forestry, Mining and Energy Union ACT branch secretary Jason O'Mara said installing the concrete panels was a "high-risk" exercise.

He called on the ACT government to introduce legislation to make the job safer for construction site workers, as well as pedestrians passing work zones.

"We need to act now, and not once someone has died from this," Mr O'Mara said.

"We would like to see a fast-tracked code of practice put in the regulations so it has legislative authority. We want this to happen ASAP.

"There are good codes around the country, so we are confident it could be done reasonably quickly."

The Braddon construction site at 32 Mort Street remained closed on Thursday as WorkSafe ACT continued to investigate how the 3.8-metre x 2.5-metre concrete panel became dislodged from its temporary support. Investigators spent most of Wednesday night and Thursday gathering evidence from the site.

Developer Bulum Group and builders Chase Building Group are behind the Yamaroshi apartment project.

WorkSafe ACT Safety Commissioner Greg Jones said a prohibition notice was placed on the builder following the incident.

Investigators have so far found the site did not have adequate access areas on a number of levels, and had non-compliant scaffolding. The two issues did not relate to the concrete panel.

Mr Jones expected the site would be reopened on Friday. He said the investigation into the panel falling was ongoing and further action would be considered as it progressed.

Of particular concern to WorkSafe was the scaffolding, Mr Jones said.

This matter will be looked at closely as part of the investigation and Id encourage all industry to thoroughly check their scaffolding to ensure that it fully is compliant and safe, he said.

Plans were under way to remove the fallen panel from a neighbouring smash-repair business.

Mr O'Mara said "a lot" of things could go wrong with installing the precast panels if each step wasn't done "100 per cent" correctly.

"Building quality, building safety, doing the right things: they arent exclusive, they work together," he said.

"This incident is just a reminder of how dangerous the construction industry can be. You can see the damage, people don't come out of incidents with concrete panels."

At the Braddon site, Mr O'Mara said he understood the panel fell "somewhere between the temporary panel and permanent bracing being placed".

"There has obviously been a breakdown in the process somewhere," he said.

Mr O'Mara said the union was working with WorkSafe ACT to complete guidance notes for high-risk practices. They had completed notes for the use of power cranes, however those notes are not enforceable until legislated.

The pressure on construction companies to meet tighter completion deadlines contributed to the issues of quality and safety faced in the industry, Mr O'Mara said.

Safe Work Australia data shows the second biggest cause of fatalities in the workplace in 2017 was being hit by a moving object. In that year alone, 35 people across the country died at work from being hit by an object.