A homeowner whose property has been fingered as the likely source of a smell that made Carterton school children sick has labelled the situation a "case of hysteria".
Police today revealed the mystery substance that made the children violently ill last week was likely a fresh, hot load of mushroom compost dumped on a property adjacent to the South End School about 1pm on Friday.
About one cubic metre of the compost was dumped in the vege garden of Illya McLellan.
McLellan said there was a suspicion that the illness outbreak had something to do with the compost as early as Friday evening.
"The police came over and asked me about it," he said. "The thing I find hard to believe is there is any association between the sickness and the compost. I think this is a bit of a case of hysteria."
Posting on the school's Facebook page, McLellan wrote that it showed how something small like an unpleasant odour could blow up into a national incident.
"Kids smell compost, some kid says he saw a plane fly over the school with stuff dropping out of it. Some kid feels sick. Some other kid feels sick. The plane did fly over the school, but was an aerobatic plane flying to the South Island. The compost did stink, it's compost."
Ten children ended up being hospitalised with dizziness and vomiting on Friday. All were eventually discharged.
The manufacturer whose compost the children to fall ill has labelled the subsequent fallout an overreaction.
Parkvale Mushrooms owner Clive Thompson said it was only about a cubic metre of compost.
"It was just someone putting a bit of compost on their garden ... it does smell a little bit when you move it but that soon dissipates."
Thompson has been selling the compost for 52 years and has never had an issue with it.
"I think a lot of people panicked.
"In retrospect it was a bit over the top, really, for a little smell."
Wairarapa area commander Scott Miller said the children were unlikely to face any ongoing health problems.
"We have confirmed one of the neighbouring properties had a truck full of compost fertiliser delivered to that address and that compost was fresh and was hot," Miller said.
The compost was dropped at the property, which backs onto the school playground, between 1pm and 1.15pm on Friday.
Children playing near the area fell ill about 1.20pm, Miller said.
Part of the compost manufacturing process was to heat it to 80C, and when that occurred it created a sulphur smell. Police believed it was this smell that made the children sick.
"We have identified that most of the children that fell ill were from the rear of the school near the compost manufacturers," Miller said.
He said medical experts had advised that the children' symptoms were consistent with inhaling compost fumes and long-term effects were extremely unlikely.
"With the advice from the compost manufacturers - we have now spread the compost to a thin level and it no longer poses risks.
There was nothing to indicate that the compost had been handled incorrectly, Miller said.
Emergency services descended on the school on Friday afternoon after more than 50 children suffered headaches, vomiting and skin irritation at South End School.
Ten children were taken to hospital for checks and were released that night.
Police said at the weekend that they had tracked down a plane at the centre of a mystery outbreak in the Wairarapa but say they're "90 per cent sure" it didn't drop anything that caused children to get sick.
Investigators were seeking a small plane, possibly a Cessna, in their bid to trace a sulphur-smelling substance that saw 10 Carterton children hospitalised and dozens more feeling ill on Friday.
Miller said speculation the plane was the source of the outbreak arose after a student said they thought it released a "white substance". However, police had talked to a number of adults and did not think the aircraft had dropped anything.
Instead, detectives believed the vapour had come from the nearby State Highway 2, or nearby houses.
"It was airborne and was blowing through the school," Miller said at the weekend.
"However, we don't have a source for that smell ... at the moment, to be quite honest, it's a slight mystery."
Atmospheric Science Global director Jennifer Barclay told the Herald that reports of the substance smelling like rotten eggs suggested it was hydrogen sulphide (H2S).
"My knowledge suggests that it's most likely a source that is close to the ground, typical examples are sewer, rotting vegetation, anaerobic composting," Barclay said.
"Because it smells like sulphur, the fact it smells like rotten eggs is an indicator it is a sulforaphane compound.
"For the children to react the way they have it would have to be a significant source or a very nearby source."
Exposure to H2S can cause nausea, tearing and redness of eyes, with high levels of exposure causing dizziness, loss of appetite, fatigue and vomiting.