A drum of toluene was being unloaded from the back of a Tulloch Transport truck in the company's Sturdee St yard when it was punctured by a forklift about 7.15am.
Four fire appliances and the decontamination unit attended the spill and some nearby business premises were evacuated.
Tulloch manager Peter Sutherland said steps were immediately taken to prevent the solvent from flowing into drains in the yard and, in doing so, seven staff members were exposed to the chemical.
"It's volatile and flammable. That was the biggest concern. The fumes are toxic, so we evacuated everyone as quick as possible."
The seven staff were checked by ambulance officers.
Six were allowed to return to work but one was sent home for the day to recover after inhaling fumes.
All were advised by fire safety officers to remove their boots and destroy them because of concerns the chemical would react with the soles, Mr Sutherland said.
"It wasn't a good start to the day. We'll have some customers that will get their freight late today. The pleasing thing is our response to the situation was good.
"There will be an investigation into what happened . . . Something wasn't done correctly. We want to be able to learn from this."
Dunedin Central Fire Station senior station officer Bob Calder praised Tulloch staff for their quick response to the spill.
The vapour from the chemical was explosive and, because there was no wind, the fumes were building up in the loading area.
It was a potentially dangerous situation, he said.
"We had an engineering firm across the road with gas cutters and metal grinders which were an ignition source, so we had to evacuate them," he said.
Firefighters in decontamination suits spent about four hours cleaning up the spill.
