You are not permitted to download, save or email this image. Visit image gallery to purchase the image.
A Fire and Emergency New Zealand (Fenz) spokeswoman said they were called to the spill at the Duncan South facility, in Puddle Alley, about 10.25am.
Two fire appliances responded, one from Lookout Point and one from Mosgiel.
Two people had inhaled gas during the incident, she said.
St John later confirmed that three people were taken to hospital.
Some of the firefighters at the scene were wearing breathing apparatus.
The facility was about 100m from the road and close to AgResearch's Invermay campus.