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, which came back as normal.

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

He said while he wasn't at the event there was a "fair amount of anxiety."

All of the affected students had been sent home.

The pools had since closed and the Hawke's Bay District Health Board and Hastings District were investigating.