Part of the five-metre excavation pit has subsided after the weekend rainfall at a site on the corner of College Street and Eastern Valley Way. It is the subject of an ACT Work Safety investigation which is still taking place.

Construction, Forestry, Mining, Energy Union ACT secretary Dean Hall said that had the wall collapsed while workmen were in the vicinity, the outcome would have been disastrous.

"They are just bloody lucky this happened over the weekend."

The wall has been the subject of ongoing objections by the union over a lack of adequate shoring.

The union made several complaints to Work Safe over the past weeks as well as last month organizing a work shut down and protest at the site, in which the Australian Federal Police were called by principal contractor Creative Building Services to remove approximately 25 protesters.

No arrests were made.

Mr Hall said "we had concerns right from the start that the excavation and shoring on this wall was wrong."

The work was subsequently signed off on by a geotechnical engineer.

ACT Work Safety Commissioner Mark McCabe said his inspectors had attended the site on several occasion over the past few weeks following union complaints and "we sought assurances from their geotechnical engineer about the viability of the wall, which we had received".

WorkSafe has now asked the company for a new independent geotechnical report and is likely to issue prohibition notices on the site when the inspection is completed later on Monday.

Mr Hall said the wall's instability had related to a lack of required tiering and capping.

"This had been part of the original specifications so that the wall was properly secured and didn't collapse inwards."

But Creative Building Services director John Katsanevakis said the company had taken every safety precaution necessary and the incident had been a relatively minor "erosion of dirt which has shifted and fallen down because of the weekend rain – with about 30mls built up in corner''.

While the union claimed the wall supports had not been installed as a cost saving measure, Mr Katsanevakis said a different form of batter support had to be used because of the site had been surrounded by major roads and intersections and concrete anchors could not be installed.

"Physically we couldn't put the anchors in so we have gone for a different wall system and we have had a geotechnical engineer heavily involved as well as having WorkSafe out to inspect the system."

Mr Katsanevakis said there were strict protocols in place for workmen at the site during rain and the company had employed both a full time engineer on site as well as a health and safety advisor.

"If there is any rain at all the guys are stopped from working regardless, so even if they had been working over the weekend, they would have been stopped and no one would have been hurt. It is a minor amount of dirt which has slipped."

He said the AFP had been called to the site during the union protest because of the "tactics and verbal abuse used by workers against our staff".