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

"Measurements show that in lake Kivijaervi the concentrations of nickel now exceed 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 kilometres (300 miles) north of Helsinki, 900 kilogrammes (2,000 pounds) of nickel has leaked towards the lake located south of the plant.

Around 220 kg of nickel has leaked to the north of the pond.

"In addition to the nickel issue, the rise of uranium concentrations ... 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 aided by the Finnish military, has put dams in place around the affected area to contain the environmental damage, as it tries to plug the leak.

A spokesman for Talvivaara, Olli-Pekka Nissinen, told AFP that he didn't know exactly when the leakage would have been stopped completely.

"That is hard to estimate, but on Monday we told the local environment authorities that we estimate that it would take two or three days," he said.

The company then has to plan "how to clean the water, and that is something we are going to do in the next few days," he added.

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

On Tuesday the group, which has launched its own probe, was unable to explain what the source of the problem was.

"We can't see to the bottom of the gypsum pond. We have only radar photos and we don't know exactly what the hole looks like. Apparently, there is a fracture in the plastic at the bottom," Nissinen said.

In the first nine months of the year the mine produced 10.598 tonnes of nickel and 21.760 tonnes of zinc. No date has been set for the resumption of production.