A Hastings public swimming pool has been closed after 20 high school students began vomiting during the school's swimming sports this morning.

Taradale High School principal, Stephen Hensman, said 20 students become sick shortly after entering Frimley Pools at about 9am.

Another 20 swimmers then felt nauseous and ill soon after.

"We made a call very early on to call it off as obviously something wasn't right," Mr Hensman said.

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It's understood a water-borne bug had been ruled out as the effects were too instantaneous.

He said pool staff tested the chlorine levels of the pool, which were normal.

About 350 students were at the pool, which spanned all years of the school.

"I wasn't there but no one likes to vomit and you just don't know what's going to happen next so I imagine there was a fair amount of anxiety."

All of the affected students had been sent home and it's understood the Hawke's Bay District Health Board and Hastings District were investigating.

The pools were now closed.