Indian officials have launched an investigation into the leak of a radioactive substance at a nuclear power plant in the south of the country.

Local police have been called in to help discover how tritium made its way into drinking water at the facility in Kaiga, about 450km from Bangalore, after 55 people had to be treated for exposure to the substance.

"Mischief is not ruled out. Investigations are on," J.P. Gupta, the director of the plant, told the Reuters news agency on Sunday.

"This incident has [in] no way affected public, safety, health and environment."

Gupta said that the 55 workers who had tested positive of tritium poisoning on Wednesday were now back at the plant.

Tritium, also known as hydrogen-3, is not dangerous when encountered externally but when inhaled, ingested or absorbed through the skin it can increase the risk of cancer.

It is an important fuel for nuclear energy production in certain types of reactors.