About 30m of the bank between Pine Hill Rd and the Gardens Ground, was churned up and two traffic marker posts were taken out as the truck and trailer gouged into the bank.
Mr Horn said he saw only the end of the accident. He heard a loud bang as the truck and trailer rolled, eventually coming to rest in front of the botanic garden fence in Great King St.
''He did so well to avoid crashing into anything or killing anyone.
''There were quite a few people around the area. A person walking their dog wasn't too far away ...
''It could have been so much worse.''
Mr Horn said he reacted without thinking in dragging the driver from the cab.
''Some people may say that's not what you should do because the guy might have been seriously injured - but you just react when you see something like that.
''I just thought the truck could burst into flames at any moment.''
A witness, who did not want to be named, said she was driving home, northbound in Pine Hill Rd, when she saw the truck on its side.
As a former firefighter, she knew what to do. She called emergency services and helped to comfort the driver before ambulance staff arrived.
St John staff arrived shortly after and the man was loaded into the back of an ambulance.
Police closed part of Great King St near the botanic garden and diverted southbound traffic on Pine Hill Rd.
Senior Sergeant Ben Butterfield, of the Southern District Command Centre, said it was ''extremely fortunate'' no-one was hurt, given the incident was on a busy section of road during close to peak-hour traffic.
Another vehicle or a pedestrian could have easily been taken out by the truck, Snr Sgt Butterfield said.
The police commercial vehicle investigation unit was investigating the crash, which would include a mechanical inspection of the vehicle.
It was ''too early'' to confirm whether brake failure was a factor, he said. A crane was used last night to right the truck and it was removed from the scene by about 9pm.
