Emergency services are responding to a potential hazardous chemical incident at the Christchurch Community Corrections office on Ensors Rd in Christchurch on Friday afternoon.
Police say at least two staff complained of burning throats, vomiting and struggling to stay awake after exposure to an unknown substance.
A St John spokesperson says two patients are in a moderate condition.
"The patients appear to have been exposed to a hazardous substance which has yet to be identified," they say.
"Up to 30 other people at the same location are not showing any signs of contamination and are not, at this stage, requiring medical attention."
A witness on the scene told Newshub someone from the building ran across the road looking for a defibrillator.
The area around has been cordoned off with some surrounding roads also closed.
A Newshub reporter on the scene says there are at least six fire trucks are on the scene along with a decontamination unit.
The Department of Corrections says their focus is on supporting the wellbeing of their staff affected.
Newshub.
