Five people have been taken to hospital in Perth after a leak of chlorine gas at James Packer's Crown Casino complex at Burswood.
The Department of Fire and Emergency Services said two liquid chemicals had been mixed by contractors to produce the toxic gas underneath the main casino complex.
No casino customers are believed to have been affected.
DFES spokesman Allen Gale said after the area was evacuated, five people were taken to hospital for treatment, and a further 15 had been affected by the fumes.
"Chlorine gas like this would spread very quickly, and people closest to it would get a bit of skin irritation from it as well as the moist parts like the eyes and nasal tissue," Mr Gale said.
"It would be quite uncomfortable at the time, so fingers crossed there are no lasting injuries and we are pretty sure that is the case because it was dealt with very quickly."
Emergency crews will remain on site into the afternoon to ensure the site is safe.