A Fire and Emergency service spokesman says he does not believe the mystery substance affecting dozens of children at a Wairarapa school came from a plane.

It had been widely thought that a chemical came from a plane flying over South End School in Carterton.

But Fire and Emergency incident controller Brett Lockyer told Newstalk ZB that he didn't think that was the case.

"We've had some reports about that," Lockyer told Larry Williams.

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"But that's been unsubstantiated at this stage and we don't believe it's come from a plane ... we just don't know where it's come from. That's a mystery to us at the moment."

"Some students have experienced some nausea and minor effects of the substance. Around 15 children that have been affected have been transported to Wairarapa Hospital for further assessment.

"And we have a further hundred students at the school and we have just triaged them.

"We are going to go through a decontamination process with those students as a precaution and then we will assess those ...

"Most likely they will be reunited with their parents and head back home."

He said the children began to experience the effects of the unknown substance within minutes of their being exposed to it.

"... the way the children are coping at the moment it seems to be a short term effect more than long term."

Earlier this evening, a Wellington Free Ambulance spokeswoman said 50 children were being treated by health workers.

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Forty who had "very minor" symptoms were being treated at the scene.

Ten other children who were suffering moderate symptoms had been taken to Wairarapa Hospital.

The spokeswoman did not know what symptoms the children were experiencing.

A decontamination unit has been set up at the Wairarapa school.

One student reportedly told school staff they had seen a plane dropping a substance near the school, and a strong smell of rotten eggs was reported in the playground.

At least five ambulances and eight fire appliances are at the school.

Carterton senior fire officer Wayne Robinson said 33 children had shown signs of illness, including vomiting, and four would be taken to hospital.

All staff and students will be showered and decontaminated before they leave the school. Parents have been asked to bring a change of clothes for their children.

Photo / Wairarapa Times-Age
Photo / Wairarapa Times-Age

No students are currently allowed to leave the school grounds, and no one is permitted on to the grounds.

South End School is a primary school with 140 students from Year 1 to Year 8.

Warning - at approx 1pm today there was a very strong smell ( strong amonia rotten eggs)in our school playground - a...

Posted by South End on Thursday, 20 September 2018

"Teachers have confirmed they noticed an unpleasant smell which quickly disappeared.
Inquiries are continuing into the source of the smell," Wairarapa police Inspector Scott Miller said in a statement.

"As part of those enquiries police are going door to door in the area checking on residents' welfare and trying to locate the source of the smell.

"The school grounds are also being searched."

Wellington Free Ambulance Service has an area manager at South End school assessing the situation, plus four ambulances attending to the students

More to come.