A leach tank with 1,450 cubic metre capacity failed at Ranger Mine spilling mud, water, ore and acid.
Workers discovered the breech at 1 a.m. Saturday, a hole in the side of Leach Tank 1. A crane attempted to block the hole but was toppled when the tank gave way. No personnel were injured.
Energy Resources Australia (ASX:ERA), Ranger Mine's operators, said the spill was contained.
"Containment systems stopped the flow, and this has meant there is no impact to the surrounding environment," said ERA General Manager Operations Tim Eckersley.
"ERA is focusing on clean up and recovery, and the protection of the environment and the health and safety of our people remains paramount."
The company said that it is ". . . confident that the nearby Kakadu National Park will not be impacted as a result of this incident." The park, which surrounds Ranger Mine, is a UNESCO World Heritage Site.
Production at the company has ceased. Federal and territorial regulators are investigating the incident.
The company said that ". . . is taking this incident extremely seriously."
On its Facebook page the Gundjeihmi Aboriginal Corporation called the accident a "catastrophic failure."
The company opened 9.62% lower at A$1.17 .