The man, a worker for Bowser Fire Protection, fell down the shaft at about 12pm.

He was conscious after the fall and was able to call out for help.

Intensive care paramedics treated the man for chest injuries and conveyed him to Canberra Hospital, where he remained in a stable condition this afternoon.

WorkSafe ACT investigators have visited the site today, but ACT Work Safety Commissioner Mark McCabe said it's still unclear how the fall happened.

Mr McCabe said nobody on site witnessed the incident and investigators are yet to speak with the injured man.

“He appears to have been working by himself at the time,” he said.

“We’ve put a prohibition notice on any further work in the vincinity of the incident while we do our investigations.”

Mr McCabe said the man’s injuries could have been much worse, given the distance he fell.

“It sounds like he’s been very lucky,” he said.

WorkSafe ACT have investigated several incidents at the Nishi construction site since work began there.

In July, a fatality was narrowly avoided and a taxi significantly damaged, after chunks of concrete were accidentally shot onto busy Civic streets from the site.

Mr McCabe said in this instance it's still unclear whether anyone was at fault, but given the number of incidents at the site a broader investigation was a possibility.

“I know that we have been there a number of times,” he said.

It comes less than a month after a housepainter was taken to hospital with back injuries and broken bones after a four-metre fall from a ladder at a house he was painting in O'Connor.

The have been four fatal accidents on Canberra worksites in the space of about a year, prompting union protests calling for better safety regulation of the ACT industry last month.