Emergency services were called to a commercial premises in Branston Street at about 8.20am on Friday.
St John said one person was taken to Christchurch Hospital in a serious condition.
It was the third workplace incident in Christchurch in just three weeks.
On Tuesday, a person was found dead at a commercial premises on Carmen Road, also in Hornby.
Near the start of the month, a person died at an industrial site in Harewood.