UPDATED: 5.27 PM: Police have 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 yesterday afternoon.

Wairarapa Area Commander Inspector Scott Miller told media today that detectives had since traced eight planes, including one that flew over the school around 2pm.

They were yet to speak to the pilot, but CAA had interviewed them and they were almost certainly ruled out.

"At this point that plane, although overhead, didn't have anything to do with this incident. we are 90 per cent sure it wasn't involved," Miller said.

He 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. "However, we don't have a source for that smell....at the moment to be quite honest it's a slight mystery."

Police would now go door-to-door to try to find the source of the smell.

A huge emergency response was triggered by the incident, which saw more than 50 children suffer headaches, vomiting and skin irritation at South End school after the sulphur smell was reported.

Ten children were taken to hospital but later discharged.

During the incident, children and teachers were locked at the school until 8pm so they could be decontaminated, leaving parents worried and the community stressed.

"It was scary not being allowed in to see my baby," said mum Samantha Cadwallader. "[I] hope the families that had ill children are all doing ok and the children feel better real soon."

Mum Megan Syskes said she was "so relieved" to have her son back safe and sound, and thanked emergency services, the mayor, and the school staff for keeping the children comfortable.

"Hug your babies extra hard tonight, I know I have," she said.

Emergency services have described the incident as "strange" and "weird" - and say although all the children have now recovered, they wanted to err on the side of caution until the cause could be found.

"But we don't have much to go on, which makes our jobs so much more difficult," said Fire and Emergency incident controller Brett Lockyer. 

"Normally we know what we're dealing with, what we have to do and how to do it. But we're just coming to a dead end with most of the lines of inquiry we're going down at the moment."

Lockyer said scans of the children's clothing and searches at the school had been fruitless so far.

"We had a good scour around the land, around the playgrounds, trying to find remnants of granules or materials," he said. "We couldn't find anything last night so it's unlikely there will be anything there today."

Police had also been investigating at the site, and taking photos.

Health authorities said all of those taken to hospital were released last night and there had been no new cases. 

Board chair Brian Chin thanked all those involved. He said parents had been given extra information about what to do if their children relapsed.

The school was now open again.

There would be a community meeting on Wednesday to debrief about the situation.

Anyone living close to the school with any further information is urged to get in touch with Masterton Police on (06) 370 0300.