Xstrata subsidiary McArthur River Mining is facing a $70,000 penalty for spilling thousands of litres of diesel two years ago.
The Northern Territory miner is being fined for leaking almost 28,000 litres of diesel at the zinc mine site near Borroloola in May 2011.
The Darwin Magistrates Court handed down the verdict on Friday saying the leak resulted in material environmental harm, the ABC reported.
Magistrate John Lowndes said the impact was “fairly serious” but highlighted the company allotted $250,000 cleaning the site after the leak, indicating they had learned their lesson.
He added McArthur River Mining was a good corporate citizen.
Australian Mining reported last year the company could face a maximum $1 million fine for the diesel leak.
The leak was discovered from a pipe connected to a storage tank and the Government said the valve had been open for two days.
Xstrata tried to allay environmental fears over the expansion of the zinc mine after the incident.
The Environment Centre NT had asked Xstrata to demonstrate it has a strategy in place to avoid contamination of the McArthur River.
A company spokesman said it had spent $750,000 on cleaning up the site after the spill.
The company was given a 28-day deadline to pay the penalty but defence lawyer Anthony Crocker said it would require more time.
But the spokesman said it would meet the penalty deadline.