A large portion of the Geocon construction site for its new "Southport" apartments gave way over the weekend.

A spokeswoman for Geocon says run-off from heavy rainfall caused excavation work to slip, as it was designed to do. Water had pooled around three or four main areas, which had caused the shutdown, the spokeswoman said.

"Nothing untoward. They did close the site to rectify it. Worksafe inspected the site and were happy with the progress," she said.

Construction Forestry Mining Energy Union ACT secretary Dean Hall says had the collapse happened when people were working on site, there may have been serious injuries, or even deaths.

Mr Hall said 50 per cent of the excavation work had given way because it had not been properly supported with piers.

Mr Hall rejected Geocon's assertion his safety concerns related to wage bargaining on behalf of building workers. "The bargaining outcome didn't cause the collapse," Mr Hall said.

Geocon's spokeswoman said the company had introduced a non-union agreement with its workers, which had made the union "hopping mad" because Geocon was the biggest company in the ACT to implement such an agreement.

The southbound lane of Anketell Street in front of Bunnings Tuggeranong was closed and traffic diverted along Reed Street.

A reduced weight limit of two tonnes on the road, and reduced speed limits were introduced.