Authorities are hunting for the source of the supposed chemical incident that affected dozens of people in the Wairarapa town of Carterton and put 10 children in hospital.
Residents reported a strong smell-like sulphur today and a school was locked down, with no-one allowed in or out, while the children were assessed and put through a decontamination process.
Carterton's South End School posted on social media that there was a very strong smell of "ammonia rotten eggs" in the playground at around 1pm.
"... a plane flew in a southward direction and one student had seen 'stuff' coming out of the plane - so we assume it was a fertiliser of some sort."
Several students felt queasy and they were made to wash their faces and drink plenty of water.
The range of symptoms included nausea, vomiting, stomach pain and a slightly raised temperature.
At least five ambulances and eight fire engines went to the school.
Wellington Free Ambulance said 40 children who had "very minor" symptoms were treated at the school by paramedics.
Ten others with moderate symptoms were taken to Wairarapa Hospital in Masterton.
Fire and Emergency said about 100 children at the school were triaged and put through a decontamination process. Parents were asked to bring a change of clothes for their children.
By 7pm, the last of the 10 who were taken to hospital had been discharged, a hospital spokeswoman said.
She expected the regional public health service would be involved in trying to locate the source of the problem.
Police said inquiries were ongoing to find the source. They were door knocking in the area and investigating the school grounds.
A Fire and Emergency spokesman, commenting on reports of a substance coming from a plane, said, "... 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."