St John Ambulance treated a number of children after a chlorine gas leak at Arena Joondalup.
Just after midday the Department of Fire and Emergency Services was notified of the leak, which was coming from the pool area. About twenty minutes later the area had been evacuated.
Channel Seven reporter Rob Scott told Fairfax radio about 30 people had been affected by the leak.
He said only one person required oxygen treatment.
No one affected needed hospital treatment.
Police and the Department of Environment and Conservation officers also attended the scene.
The venue is expected to be closed for several hours.