Worksafe ACT has since closed the worksite because of the incident and says it won't be reopened until it is declared safe.

The driver was found lying next to his tip truck after the incident at the dump site on Boldrewood Street and Barry Drive.

It is believed he suffered an electric shock after his truck came into contact with overhead powerlines while dumping material at the site. He was transferred from Calvary Hospital to St George Hospital's intensive-care unit in Kogarah, where he was in a critical but stable condition last night.

ACT Work Safety Commissioner Mark McCabe said: ''I had heard that the worker was to be brought out of his coma today and there were grave concerns as to how that was going to go.

''These accidents, when they do happen, there are usually very serious injuries.

''They happen far more frequently than they should.''

Mr McCabe said the back of the tip truck, which has burn marks along the top, was found fully extended and just inches from a powerline above after the incident.

There was a power outage in the area at about 3pm.

It is believed the man was outside the truck when he suffered the electric shock.

''It appears he got out of the truck and at some point the truck became live and when he touched the truck that's when he got the shock,'' Mr McCabe said.

Worksafe ACT has been tasked with finding out why the truck was operating below powerlines and whether workers were authorised to take material into the unsafe site.

No witnesses have come forward and the truck has been impounded by WorkSafe ACT for testing.