A public pool is Hastings has been closed while authorities investigate a vomiting outbreak among students attending a school swimming sports day.

About 20 Taradale High School students suddenly became sick at the Frimley Pool on Wednesday morning during championship races being held as part of the sports day.

School principal Stephen Hensman said a similar number of students reported feeling nauseous, although they were not physically ill.

All appeared to be recovering by Wednesday afternoon and testing of the pool water had not shed any light on what might have caused the students to be sick.