The children were taken to Southland Hospital as a precaution and later discharged. About 20 others were treated at the theatre.
Invercargill City Council venues manager Kate Feaver said today the Fire Service had checked the theatre and ruled out any chemical or gas leaks.
''They did a thorough job, and it's good to know that it was nothing to do with the building.''
About 240 children from seven schools were on stage rehearsing for their performance in the city's Big Sing week of choral music when one child fainted, followed by several others.
Ms Feaver said after talking to emergency services personnel and teachers it seemed there was ''nothing sinister'' about the incident.
''It started off with a child who had had flu fainting... which started a chain reaction. It was a bit of a Mexican wave.''
She believe the fainting was caused by a combination of factors including children ''getting quite excited'' about their performance and being on stage, and some possibly being dehydrated.
Emergency services rushed to the theatre about 1.20pm. Police closed one block of Esk St at the rear of the theatre while some St John staff donned white protective suits.
Ms Feaver said the response ''possibly did add to the drama of it'' for the children.
''It was a bit like a CSI scene. There were people in uniforms, people in white suits, flashing lights.''
The opening night of the Big Sing went ahead on Tuesday night as planned, she said, with no children fainting.
