More than 50 people have been treated for chemical poisoning at the Craigie Leisure Centre on Thursday after inhaling chlorine.
The centre was evcauated and closed just after midday after a number of swimmers became ill.
City of Joondalup CEO Garry Hunt said an investigation was underway to determine what happened.
St John Ambulance confirmed the swimmers were treated for chroline inhalation. Most of the swimmers were treated on-site.
Four children were taken to Princess Margaret as a precaution.
Two adults were taken to Sir Charles Gairdner and two to Joondalup Hospital for further check-ups.
More to come