Japan has raised the severity of the latest leak at the Fukushima nuclear plant, now describing it as a "serious radiation incident".

The plant's operator, TEPCO, has revealed 300 tonnes of highly radioactive water has leaked from a tank at the plant.

The water is so contaminated that TEPCO says a worker standing near it for just one hour would receive a dose five times above what it regards as the safe annual limit.

The company yesterday revealed the highly radioactive water had leaked from a storage tank and had formed puddles on the ground, with fears it could contaminate groundwater flowing into the Pacific.

After declaring it a level one incident on the International Nuclear Event Scale, Japan has now raised it to level three, which is defined as a serious incident involving severe contamination.

The company has faced a growing catalogue of incidents at the plant including several leaks of radioactive water, more than two years after the worst nuclear disaster in a generation which was triggered by a huge quake and tsunami in March 2011.

The company - which faces huge clean-up and compensation costs - has struggled with a massive amount of radioactive water accumulating as a result of continuing water injections to cool reactors.

The embattled utility in July admitted for the first time that radioactive groundwater had been leaking outside the plant and this month started pumping it out to reduce leakage into the Pacific Ocean.

The problems have led the Japanese government and its nuclear regulator to say they would get more directly involved in the cleanup at Fukushima plant, which had largely been left in the hands of the company.

While no one is officially recorded as having died as a direct result of the meltdowns at Fukushima's reactors, large areas around the plant had to be evacuated, with tens of thousands of people still unable to return to their homes.

ABC/AFP