Two hundred litres of hydrochlorite acid which spilled from a truck in Otago's Kawarau Gorge caused no environmental damage and actually cleaned the road, the Fire Service said.

State Highway 6 was closed for more than three hours yesterday after one of two drums fell from a Mainfreight Transport truck which was taking the low-grade acid from Cromwell to Queenstown.

The drum had been dragged about 300m down the road after a strop holding it onto the truck broke, Arrowtown fire chief Garry Hall said yesterday.

"Virtually all it's done is clean the road," he said. "It left a clean white strip down the road."

By the time the truck driver realised the drum had come loose it was almost empty. He called emergency services when it started smoking after he had rolled it onto the grass verge.