A faulty transformer has been blamed for a gas leak at a Hobart zinc plant which forced thousands of residents and students indoors.
Emergency crews gave the all-clear after the incident at the Nyrstar smelter at Lutana about mid morning.
Fire crews, police and the Environment Protection Authority are investigating the release of sulphur trioxide and sulphur dioxide.
Parts of the smelter were evacuated and residents of Lutana, Cornelian Bay, Risdon and Lindisfarne were warned to stay indoors in case winds blew the potentially dangerous chemical into their suburbs.
Nyrstar's general manager, Jeremy Kouw, says a transformer in the smelter failed about 9:00am, cutting power.
"The issue is that we get an internal power spike and that's basically tripped a lot of the motors," he said.
Managers released a cloud of sulphur dioxide and sulphur trioxide to protect employees after the power failure stopped an acid plant.
Mr Kouw says for the safety of employees, it was important to keep some gases flowing through the plant.
"For about 30 minutes this morning we had a visible emission of processed gas after the power failure," he said.
"That's a mixture of sulphur dioxide and sulphur trioxide. It's mainly sulphur dioxide that's been emitted to atmosphere."
The company has acknowledged a series of loud sirens which followed the leak added to community angst.
A loud bang was heard across the River Derwent shortly before 9:00am. A gas plume followed and was visible for about 30 minutes.
Mr Kouw says the company restarted the affected machinery, causing loud sirens to ring but says they are part of operational safety.
"The siren just alerts people to the fact that a piece of rotating equipment is about to start."
The spill also forced 2,500 school students indoors.
The Education Department's Judy Travers says affected schools responded quickly.
"As soon as the police advised us, we rang every principal and school office in a three kilometre radius of the site, as well as schools on eastern shore as a precautionary measure."
"Our advice for them was certainly to keep all students in at recess and to have our windows shut and all air conditioning turned off."
There were no reports of injuries.
A chemical expert says the effect of the gases can be severe.
Professor of Chemistry at the University of Tasmania, Paul Haddad, says if anyone had been walking past at the time, it would not have been pleasant.
"[Sulphur trioxide] its problems relate to the fact that as soon as it contacts water it becomes sulphuric acid," he said.
"So sulphur trioxide is the precursor to acid rain. If you breathe in sulphur trioxide, as soon as it contacts your nose, or your moth throat and lungs, it forms sulphuric acid in situ, and it's extremely acrid and unpleasant and dangerous."
Two years ago, one worker at the plant was affected by sulphur dioxide gas when a power surge triggered a shut down.