Bystanders who saw the incident near Silver Fern Farms' Finegand freezing works said the truck's driver, a woman in her 30s, appeared to lose consciousness at the wheel in a 30kmh roadworks safety zone.
Allan Corsan, of Invercargill, a Fulton Hogan contractor sealing road patches at the location, said the woman was a Fulton Hogan colleague from Invercargill, who was delivering gravel to the site at the time.
Mr Corsan said his colleague appeared to have fainted.
''She was about to pull up to reverse into a layby here, but instead she just sort of slumped over the wheel, carried on going, and slowly drifted off the other side of the road about 100m away.''
Fortunately, he said, the 30kmh zone had been closed off to allow the truck to manoeuvre, avoiding any collision with other vehicles.
The resulting low-speed crash into swampy ground about 3m below the road verge appeared to have left the driver dazed, after regaining consciousness, but without obvious physical injury.
Ambulance, police and two fire appliances from Balclutha attended the incident.
Police said a woman had been treated for minor injuries at the scene, and taken to Clutha Health First in Balclutha for precautionary observation following the incident, which was believed to have been caused by a ''medical event''.
No other vehicles were involved in the incident, which police would continue to investigate.
