Workers were pouring concrete on the east-west section of the new bridge when a cracking sound was heard.

The bridge buckled beneath them seconds later, taking construction workers standing on the upper section with it.

Some workers scrambled to safety in time but one was left suspended in mid-air after he grabbed hold of a cement truck hose.

CFMEU ACT branch secretary Dean Hall said, ''They heard a crack, there was a three-second delay and then it gave way.''

A worker with lower leg injuries was trapped in a confined space for about 15 minutes before being rescued by fire crews.

Most others were able to walk to waiting ambulance crews for treatment. Six workers were treated on-site for minor injuries by paramedics.

Those taken to hospital had injuries ranging from ''broken arms and legs'' to suspected spinal injuries.

A cement pourer escaped uninjured after he grabbed on to a concreting hose as the bridge collapsed beneath him.

For more on this story, see the print edition of today's Canberra Times.