A 25-year-old man suffered serious injuries after falling three metres at a work site in Gungahlin on Monday morning.
An investigation by Worksafe ACT and union officials is underway into how the man, who had been working on the upgrade to the Gungahlin Marketplace shopping centre for three weeks, was injured.
CFMEU ACT secretary Dean Hall said the man was reinforcing a concrete slab with steel cable when he stepped on a sheet of formwork ply which acted "like a see-saw".
He then fell three metres onto a concrete slab, suffering lower back and pelvic injuries.
He was discharged from the Canberra Hospital shortly before 4pm.
Other workers from the construction company, Amisco, provided the man with first aid before paramedics arrived.
An ACT Emergency Services spokesman said that after a "difficult extrication", the man was stabilised at the scene before being taken to Canberra Hospital for further treatment.
Mr Hall said the union would provide counselling and support to workers on-site.
The union would also offer support to the man and his family "at an appropriate time", he said.
WorkSafe ACT has been approached for comment.