Fire services were checking the St Asaph St address as well as the probation facility on Ensors Rd.
Police had cordons in place nearby and were diverting traffic, the spokeswoman said.
Two people were being treated by St John at the scene, said spokeswoman Noreen Hegarty.
"Two patients presented with what appears to be a hazardous substance, unidentified at this stage," she said.
Thirty other people were at the Ensors Rd address but showing no signs of being affected.
Emergency services were staying at the scene until the substance was identified, Hegarty said.
Ensors Road has currently been closed northbound from Brougham St by emergency services, a Christchurch Transport spokeswoman said.
Motorists are advised to use an alternative route at this time and to expect delays.
Assistant area commander Steve Kennedy said emergency services were packing up and leaving the scene shortly before seven.
Fire crews had been called in by St John this afternoon.
No traces of chemicals had been detected on the clothing of the people who became sick, or on their car.
It was possible they had been experiencing an allergic reaction, Kennedy said.
"We've had a couple of people who have returned to a workplace and they've been suffering symptoms of either exposure to a chemical or severe allergic reaction to something," he said.
They had been decontaminated as a precaution and taken to Christchurch hospital in a moderate condition.
