Twenty students have become ill during a swimming sports day at a public pool in Hastings.
A Taradale High School pupil became ill and vomited at Frimley Pool this morning.
Within 30 minutes nearly 20 more children became nauseous, forcing all students to return to class and the pool to be closed.
A chlorine test was carried out by pool staff by 10.40am and came back as being in the average range. The regular 8am test was also within the same range.
Further water testing has been referred to a commercial water testing agency. Initial results tests were expected on Thursday.
Principal Stephen Hensman said the sick students were feeling much better and the school was at a loss to explain the cause of the illness.
"Staff responded really quickly and sensibly by leaving the pools, but we do not know if the illness was in fact related to the pool."
Hastings District Council pools manager Alice Heather said it was expected that Frimley Pool would reopen on Saturday.
The Hastings Weet-Bix Kids TRYathlon, due to be held at the pool and adjoining Frimley Park on Thursday, has been reduced to a duathlon.
Sanitarium spokesman Peter Levett said the health and safety of its participants was the company's primary concern, and for that reason the decision was made to cancel the swim leg of the event.
"As soon as we were made aware of a potential issue, a decision was made to change the event to a Run-Bike-Run."
The Hastings District Council and the Public Health Unit were also investigating the cause of the illness.