The gas was detected by the monitoring system in the chiller plant at about 3pm that day.
Once the gas was detected emergency procedures were activated on-site and the area evacuated, with Queensland Fire and Emergency Services also attending the scene.
No injuries were reported and the company has started an investigation.
The Queensland Mines Inspectorate told Australia's Mining Monthly it was notified at the time that ammonia gas had leaked from equipment at the chiller plant but luckily no injuries were reported.
The QMI said it had been to the site of the incident to investigate, and the investigation was closed.