Five people have been taken to hospital after being exposed to sulphur dioxide at the Ravensdown plant in Dunedin.
A Fenz spokesman said they were notified at 10.13am of an explosion at the plant on Ravensbourne Rd.
He said when crews arrived, there was no fire, but five workers had been exposed to sulphur dioxide.
Sulphur dioxide can irritate the nose, throat and lungs and cause respiratory problems, according to the Ministry for the Environment.
A St John spokesman said two ambulances and a manager attended.
One person was moderately injured while four people had minor injuries.They were all taken to Dunedin Hospital.