Groundwater radiation levels at Japan's crippled Fukushima nuclear plant have soared near a tank that leaked 300 tonnes of toxic water in August, struggling operator Tokyo Electric Power said.

TEPCO said water samples collected from a well on Thursday contained 400,000 becquerels per litre of beta-ray emitting substances, the highest reading since the nuclear accident was triggered in March 2011.

Samples in previous days had been showing levels of just 60 to 90 becquerels per litre.

The reading for radioactive tritium also shot up to a record high of 790,000 becquerels, TEPCO said.

The leaking storage tank was discovered in August and TEPCO said measures taken since then had included transferring water from the tank and removing the surrounding soil.

"Other wells are not showing such a high level. We need to take additional measures for the areas around this well," Masayuki Ono, TEPCO spokesman, told a press conference on Friday evening.

TEPCO has removed most of the soil contaminated by the August spill, but might have failed to collect some radioactive materials, Ono said.

Heavy rain over recent weeks could have washed contaminated materials into the well, he added, saying TEPCO would remove the toxic soil and groundwater.

Meanwhile, Prime Minister Shinzo Abe visited a fishing port in Fukushima Saturday and ate locally caught seafood to showcase its safety.

Abe visited a fishing port in Matsukawaura, some 40 kilometres north of the crippled plant, where fishermen are conducting "experimental operations" since last month to catch selected marine species such as octopus that have not been significantly affected by the nuclear accident.

"We will give accurate information to the public to erase" economic harm done to the area due to wrong understanding of Fukushima seafood, Abe told local fishermen.

"I wish people across Japan will come to know that it is safe and tasty," he said.

A huge earthquake and tsunami in 2011 devastated Japan's northeast coast and sparked reactor meltdowns at the Fukushima plant by knocking out its cooling systems.

TEPCO, which has come in for growing criticism over its handling of the aftermath of the disaster, has poured thousands of tonnes of water onto the reactors to keep them cool in an ongoing operation.

Radioactive water is being stored in around 1,000 tanks but the company has admitted contaminated liquid has made its way into the sea.

Tens of thousands of people who were evacuated from the Fukushima region are still unable to return to their homes, with scientists warning some areas will have to be abandoned forever.