Police say a plane was likely not involved in the incident at a Carterton school that saw 10 primary school children taken to Wairarapa Hospital after falling ill yesterday.
Emergency services were called to South End School yesterday afternoon after reports of an unpleasant smell.
Paramedics treated 40 other people - children and adults - with minor symptoms and over 100 people had to go through a decontamination process.
Police searched the school grounds this morning but were unable to find the source of the smell, Wairarapa Area Commander Inspector Scott Miller said.
"Witnesses have described seeing a grey and white or red and white plane, possibly a Cessna, flying near the school between 1.30 and 2.30pm yesterday," Inspector Miller said.
After tracking down the plane and its pilot, Inspector Miller says there is no evidence to indicate the plane is the source of the smell.
"The initial call said a student saw a plane fly over the school and drop a white substance, after speaking to adults they confirmed that nothing came from the plane."
Inspector Miller says after speaking with residents it appears the substance came from the road and wafted over the school.
He says it's still a "mystery" and investigations are ongoing, including looking at local factories.
There are no ongoing effects for the children who became ill and the school will be open on Monday with a police presence.