Ambulance workers treated 18 people following a chlorine spillage at a Thames school.
Fire and Emergency NZ said five crews attended the spill and cordoned off an area to make it safe.
A spokesman said the school caretaker inhaled some gas and took himself off to hospital before fire crews arrived.
"This came as a result of mixing two chemicals together," he said.
"It wasn't a large spillage. Crews are still there cleaning it all up."
A St John spokeswoman said 18 people in total were treated in a minor condition at the scene. No one needed transporting to hospital.
The incident took place at Pārāwai Primary School on Lowe Ave.
In a post on the school Facebook page, acting principal Erin Bergersen said "an incident with chemicals occurred within the school pool area today late afternoon, but not in the school swimming pool itself".
"Emergency services attended. Fire and Emergency NZ checked and cleared the area. All children and staff who were in the vicinity at the time, were medically checked, cleared and sent home by ambulance staff," the post read.
"One staff member self presented to medical staff as a precautionary measure.
"We appreciate the patience and understanding of our parent community during this time as we took precautionary steps to ensure everyone's safety.
"Everything is now safe. School will be open tomorrow as normal. Please contact the school office should you have further queries."
Ministry of Education spokeswoman Katrina Casey said it was notified this afternoon of an incident at Pārāwai School.
"We understand that emergency services have been called.
"The school is working to ensure the safety of its students and staff and is in the process of updating family and whānau. We will provide any support the school requires," the deputy secretary of sector enablement and support said.
"We encourage schools to contact the relevant experts in an emergency. Depending on the circumstances this could be Civil Defence, the local District Health Board or emergency services.
"Schools produce their own emergency management plans. We have advice on our website though to help schools prepare."
The Herald has approached the school for comment.