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A "sulphur flash" at Ravensdown which left five people in Dunedin Hospital happened while a piece of old equipment was being removed.
Ravensdown communications manager Gareth Richards said it was too early to say what caused the incident, which happened in a store at its Ravensbourne site.
"We’ll be investigating what happened, but at the moment it looks like residual dust ignited during the removal of a decades-old hopper in a rock store.
"This didn’t cause flames or a loud noise, but did cause a small cloud of dust and elevated sulphur dioxide in the affected building."
A visiting contractor was taken to Dunedin Hospital for assessment following the incident, while four others, a contractor and three Ravensdown staff, were taken to hospital as a precaution.
"All are in good spirits and their wellbeing is foremost in our thoughts," he said.
Later, four of the people were discharged from hospital while one, who was "up and around talking", remained for observation, Mr Richards said.
He thanked emergency services for responding to the incident quickly and praised professional response of Ravensdown staff on site.
The suggestion from Fire and Emergency New Zealand (Fenz) that it related to welding was incorrect.
A Fenz spokesman said crews from Roslyn and Willowbank arrived at 10am.
The incident was initially reported as a medical incident, but that changed to a report of a possible explosion shortly after.
"There was a small explosion," he said.
There was no fire when crews arrived.
Five people working there were transported to hospital, not due to the explosion, but due to the inhalation of sulphur dioxide, he said.
"The scene has been left with management from Ravensdown and Worksafe."
A St John spokesman said it was alerted at 9.47am and two ambulances and a manager were sent to the scene.
Crews treated five patients, one in a moderate condition and four in a minor condition.
All patients had been transported to Dunedin Hospital.
A reporter at the scene said two Fire and Emergency New Zealand appliances attended.
One of two St John ambulances at the scene arrived and drove into the plant after 10.20am.