Toxic levels of nickel have been found in a Finnish lake after waste water began to leak from a nearby mine, the government's environmetal agency said on Tuesday.

"Measurements show that in the lake Kivijaervi, the concentrations of nickel are now exceeding the level regarded as harmful to organisms," the Finnish Environment Institute said in a statement.

Since water began flowing from a gypsum pond on November 4 at a nickel mine in Sotkamo, around 500 km north of Helsinki, 900 kg of nickel has leaked towards a nearby lake south of the plant.

In the north, around 220 kg of nickel has been discharged.

"In addition to the nickel issue, the rise of uranium concentrations in the water bodies is harmful to organisms, especially to some sensitive invertebrates and algae," the institute said.

"So far the measured concentrations of uranium are not expected to have any immediate consequences for fish populations."

The operator of the mine, Talvivaara, has stopped production and has put dams in place around the affected area to contain the environmetal damage, as it tries to plug the leak.

The police said on Monday that they had opened an enquiry into the incident.

In the first nine months of the year the mine produced 10.598 tonnes of nickel and 21.760 tonnes of zinc.