Fifteen people have been taken to hospital and over 100 people are going through a decontamination process after a chemical incident at South End school in Carterton, Wairarapa.
Wellington Free Ambulance (WFA) said those in hospital have moderate symptoms, and 40 people are being treated at the scene with very minor symptoms.
A low-flying aircraft is believed to have dropped an unknown substance over the school playground at around 1pm on Friday.
The Fire and Emergency Hazmat (hazardous materials) unit was dispatched to decontaminate people, which involves showering and putting on clean clothes.
South End school said in a post on Facebook that there was a "very strong smell" in the playground.
"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," the post said.
Seven fire crews and support vehicles and four ambulances are at the scene, and a Life Flight Westpac Rescue helicopter has been dispatched from Wellington with three paramedics on board.
Parents of students at the school were advised to go home and get clean clothes for their children because they would need to go through a decontamination process.
There is a cordon in place around the school and traffic is being diverted away from the area.
NZTA says the road is closed in the vicinity of the school on High St and at the intersection with Brooklyn Rd.
The nature of the substance remains unclear.
Low level symptoms consist of feeling nausea, vomiting and itchy eyes. WFA advises that if any conditions deteriorate, call 111.