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There were initial fears the spill could result in hundreds of tonnes of oil pollution reaching groundwater near Cockburn Sound. The refinery is on Kwinana Beach where groundwater moves towards the coast and eventually mixes with the Sound.
BP confirmed the tank is 45 years old and was last inspected 10 years ago.
A report released by the company said the leak was not detected because the rate at which the tank level dropped was below that needed to set off the alarm system.
The Department of environment is investigating whether there was a breach of licence of the Environmental Protection Act.
BP monitoring showed the contaminated groundwater had not reached the Sound.
