Fifty children are being treated by health workers after a mystery chemical fell from a plane on to South End School in Carterton.

A Wellington Free Ambulance service spokeswoman said 40 children who had "very minor" symptoms were being treated at the scene.

Ten other children who were suffering minor 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.

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.

"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

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