THE operator of Japan's crippled Fukushima nuclear plant said four tonnes of rainwater contaminated with low levels of radiation leaked during an operation to transfer the water between tank holding areas.

Tokyo Electric Power Co, known as Tepco, has been trying to contain contaminated water at the Fukushima site after it found 300 tonnes of radioactive water had leaked from a tank at the plant.

Fukushima suffered triple nuclear meltdowns and hydrogen explosions after a March 2011 earthquake and tsunami.

Heavy rain during a recent typhoon flooded one of the tank holding areas where Tepco stores excess water flushed over damaged reactors to keep them cool, a spokesman said.

Tepco officials decided to transfer the water to another holding area for tanks and during that process a worker found the leak, which the company estimated to be four tonnes and was absorbed into the ground.