The Northern Territory Environment Protection Authority says laws have failed to deal with the impact of a toxic legacy mine site near the Queensland border.

The Redbank copper mine, south of Borroloola, closed 18 years ago but continues to leak contaminated water.

The EPA says elevated metal levels have been found in waterways 35 kilometres from the site.

In a report, it says sporadic oversight from regulators, including the fact that mining commenced without an approved environmental management plan, is a key reason for ongoing environmental problems at the Redbank site.

The report says the continuing failure of mining companies to meet mining approval requirements, and the failure of regulators to hold those companies to account have been key reasons for the environmental damage.

The EPA says an environmental management plan for the Redbank site was not approved until two years after mining began.

It says commencing mining operations before getting approval should be an offence.

It has recommended making it a legal obligation for companies to comply with environmental approval conditions.

When the former owners of the mine abandoned it, they did not secure the main pit and dangerous levels of dissolved metals have been leaking from it ever since.

The cost of rehabilitating the mine area is not known but has been estimated to be between $10 million and $100 million.