On 9 November 2011 more than of ammonia was initially suspected of venting to the atmosphere from the Kooragang Island plant during a 45-minute period. The venting was due to a relief valve operating to prevent overpressure of a liquid ammonia tank. Engineering studies subsequently revised the amount down to ~. The leak was identified by firefighters responding to an alarm raised by the hospitalisation of two railway workers at who were affected by the plume of escaping gas. Six fire units and a hazardous chemicals unit were called to the chemical plant to deal with the leak of the ammonia gas. Despite initial statements by Orica that the leak posed no public health risk, two rail workers in the nearby suburb of were overcome by ammonia fumes and were taken to hospital with breathing difficulties. Less than an hour before the ammonia leak, the Environment Protection Authority announced it would take Orica to court over the hexavalent chromium leak which occurred on 8 August 2011 at the same plant.