Nine children rehearsing for a singing contest at the Civic Theatre in Invercargill this afternoon have been rushed to hospital.
It is understood as many as 12 children fainted on-stage in the theatre about 1.15pm.
Children were evacuated from the site and four ambulances as well as a rapid response vehicle was sent to the scene.
St John spokesman Ian Henderson said six children aged 10-12 have been transported to Kew Hospital emergency department in a moderate condition.
"We are assessing a number of other patients at the scene,'' he said.
It was understood to be a medical incident instead of a gas leak or a fire-related incident, he said.
Theatre manager Kate Feaver said emergency services were called to the scene after a number of school children, rehearsing for the ILT Schools' Singout, started fainting on stage.
There was nothing to indicate anything dangerous in the building that caused the children to faint, she said.
"The children were rehearsing on stage, under lights, on elevated platforms when the fainting episodes began. We understand that as many as 12 children were affected.
"An ambulance was called to the scene...Further emergency services were called as a precaution but there is nothing to indicate that there is anything dangerous in the building that has caused the children to faint.''
All the children had been evacuated from the site as a precaution and would be taken back to school by bus, she said.
Firefighters were preparing to enter the Civic to check everything was safe.
In previous years children had fainted before on the stage during this type of rehearsal so theatre staff do not heat the stage area with so many children under the lights, Ms Feaver said.
The priority for staff and emergency services was to make sure the children were safe and looked after, she said.
Mr Henderson said a total of nine children had been taken to hospital so far, he said.
The patients would be further assessed at hospital and St John Ambulance had no information on what the medical issue was.
Fire Service spokesman Brent Dunn said crews had helped police to evacuate the theatre.
Firefighters had searched the Civic and surrounding buildings for gas and had now left the scene.
"We had searched the building and surrounding buildings with a gas detector, I haven't heard what the results are, however we have left the scene.''
