THE OPERATOR of Japan's tsunami-crippled nuclear power plant has said about 300 tons of highly radioactive water have leaked from one of the hundreds of storage tanks at its site - the worst leak yet from one of the vessels.

Tokyo Electric Power Co (Tepco) said the contaminated water leaked from a steel storage tank at the wrecked Fukushima Daiichi plant. The company has not yet determined how or where the water leaked, but it is suspected to have escaped through a valve connected to a gutter around the tank.

Tepco spokesman Masayuki Ono said the leaked water seeped into the ground after escaping the piles of sandbags added to a concrete barrier around the tank. Workers were pumping out the puddle and the remaining water in the tank and will transfer it to other containers.

The radiation level of the water is about five times the annual exposure limit for plant workers.

The plant suffered multiple meltdowns following the devastating earthquake and tsunami that struck Japan in March 2011. Hundreds of tanks were built around the plant to store huge amounts of contaminated water from the three melted reactors, as well as underground water running into reactor and turbine basements.

Four other tanks of the same design have had similar leaks since last year.