The Rattray St plant was evacuated about 4pm when it was discovered 300 litres of the high-strength cleaning chemical Nitrobrite had leaked from its storage tank.
Mr Craig said the leak was contained within a bund, designed to contain spills.
Staff were alerted when they smelt gas, released when the nitric acid solution reacted with something, most likely galvanised iron pipes, Mr Craig said.
The plant's containment area, installed about a year ago, did its job well, but the placement of pipes would be investigated.
Beer production had finished for the day when the incident occurred. The plant would be open again today, although as is usual for Fridays, no beer production would take place.
Tours and plant maintenance would continue as usual.
Chemical experts would visit today to advise how to deal with the diluted nitric acid, he said. No-one was harmed by the fumes.
Decontamination and command units from Dunedin Central fire station, appliances from St Kilda, Roslyn, Willowbank and Dunedin, St John and police attended the incident.
Fire Service (East Otago) area commander Brendan Nally said firefighters wearing hazardous material suits diluted the spill to make it safe to deal with.
Disposal would be carried out by Speight's staff and the product's suppliers.
The plant was cordoned off and Rattray St closed from Dowling St to Maclaggan St, reopening about 8.30pm. Part of Dowling St was also closed to vehicles, with police controlling pedestrian access.
