Heavy rain has allowed a gold mine north of Katherine to deliberately discharge contaminated water into a nearby river.

Mount Todd mine has been inactive since its original owner went into voluntary administration in 2000.

It was sold to Vista Gold in 2006, but remains in care and maintenance.

The mine has 11 gigalitres of contaminated water containing copper, aluminium and cadmium from its historic mining activities on site, which can be legally discharged when the nearby Edith River is flowing at a high enough rate to dilute it.

Vista Gold Australia general manager Brent Murdoch said the river reached that level on January 17.

“We've been rather fortunate we've had enough rainfall and since then that it's been continuous,” he said.

“We've released a third of a gigalitre of treated water already and we're targeting a total of 1.5 gigalitres this wet season.

“It looks like we're certainly going to be able to achieve that.”

The water is treated with lime to raise its pH level before it is released, which also causes some metals to solidify and sink to the bottom of the containing ponds.

Mr Murdoch concedes that an uncontrolled discharge is possible.

“If Mother Nature decides to sit a tropical low on top of us like it did in 2011 there would most certainly be uncontrolled release of water from Mount Todd,” he said.

“If that event did happened we would be releasing water into a very flood effected system so the dilution ratios would mean there would be no risk of damage or harm to environment, as there wasn't in 2011.”