The offenders were cleaning an implement shed at Murihiku marae shortly before noon, when they found three canisters - one of which had been punctured.
Two offenders were taken to Southland Hospital by car and three others by ambulance after they began vomiting.
As a precautionary measure, Invercargill fire crews were sent to the Tramway Rd marae and found twelve more people on community service who had been exposed to the toxic material.
Murihiku runanga general manager Odele Stehlin said the marae was immediately evacuated, including those attending a Southland Community Law Centre seminar and children from the neighbouring kohanga reo.
"It has been one unusual day."
Ms Stehlin said they were able to return to the marae at 4pm.
Fire Service Southland area manager Brendan Nally said specialist units in decontamination gear worked to "bag and tag" the canisters, which had been stored in a secure shipping container.
Scientists were working to determine which chemical had caused the call-out, and a sample had been sent to a Hamilton laboratory for testing, he said.
The five people taken to hospital were released later that afternoon.
