The Parliamentary Commissioner for the Environment says 111 mining licences covering 82 mines have few, if any, enforceable environmental controls.
Jan Wright says there is a risk of New Zealand being left with a toxic legacy that would be difficult and expensive to clean up.
Dr Wright cited cases such as a toxic waste site at Mapua near Nelson and the mercury-contiminated Tui mine site in Waikato as examples of risks that could ensue.
She says she came upon the problem while researching Solid Energy's Stockton Mine in Buller, which she praised for its enviromental improvements.
But she said mining licences issued before legislation such as the Resource Management Act generally have very few environmental controls.
She says some of these mines might be operating safely, but there are few mechanisms to control them if they are not.
They cover an area three quarters the size of Lake Taupo.