The Auckland Public Health Service says children became ill at Waitakere's West Wave Aquatic Centre after breathing in chlorine gasses in the air above the pool.
Eleven children were admitted to hospital with breathing respiratory problems, after playing waterpolo at the centre on Sunday.
The excess gas is caused by poor ventilation
Public Health Medicine Specialist Dr Denise Barnfather says the levels of gas present were too low to do any permanent damage. She says at higher levels it can cause bronchial spasms, but none of the children had those symptoms.
Barnfather says the irritation the children are feeling will go away and they won't suffer any long term effects.
The children that fell ill were competing in the Under 14 waterpolo champs and affected teams are having to come up with contingency plans.
Coach of the Waikato Under 14's, Matt Fitzsimmons, has all eight of his team ill and is not certain he will have a team for the nationals, which start on Thursday.
He says it is gutting for the boys who just want to be healthy and play sport.
Fitzsimmons is talking with the event organisers about combining with another depleted team.
