On 21 July 2018, a major release of ammonia gas occurred at a Western Australian port. As part of standard operation, an ammonia tanker vessel was supplying anhydrous ammonia to a process plant. Following the completion of liquid ammonia unloading, purging of the loading arm and pipework was commenced with ammonia gas supplied by the ship via the onboard storage tank headspace. While the purging operation was underway, an operator inadvertently partially closed a line valve. As the line valve was no longer fully open, a control signal was sent to the plant emergency shut-down (ESD) valve to trip closed. This resulted in increasing internal pressure and hydraulic hammer in the loading arm, creating significant forces against the loading arm quick connect/disconnect coupler which was not sufficiently tightened for the conditions. These forces caused the loading arm to decouple from the ship’s flange and release ammonia gas. Ammonia continued to flow from the ship manifold until the ship crew manually tripped the ship’s ESD system. The release was estimated to be 1200kg over 24seconds. Five personnel present at the jetty were taken to the hospital for medical examination or assessed by ambulance services. All were released the same day.