Greenpeace says data from its radiation monitoring in the ocean off Japan's stricken Fukushima nuclear plant shows massive levels of contamination in seaweed and other marine life.

The environmental group is warning that both the environment and people are at long-term risk.

After taking samples of fish, shellfish and seaweed collected in the Pacific Ocean, 20 kilometres off Fukushima, Greenpeace sent them for analysis at independent French and Belgian laboratories.

The conservation group says the results show seaweed radiation levels are 50 times higher than official limits, while other marine samples showed high levels of radioactive caesium and iodine.

Greenpeace says it proves radioactivity is accumulating in marine life and not diluting, as claimed by Japanese authorities.

The group criticised Japanese authorities for their "continued inadequate response to the Fukushima nuclear crisis" sparked by the March 11 earthquake and tsunami.

"Despite what the authorities are claiming, radioactive hazards are not decreasing through dilution or dispersion of materials, but the radioactivity is instead accumulating in marine life," said Jan Vande Putte, a Greenpeace radiation expert.

"The concentration of radioactive iodine we found in seaweed is particularly concerning as it tells us how far contamination is spreading along the coast, and because several species of seaweed are widely eaten in Japan."

Mr Vande Putte accused Japan of doing too little to measure and share data on marine life contamination and said: "Japan's government is mistaken in assuming that an absence of data means there is no problem."

"This complacency must end now, and [the government must] instead mount a comprehensive and continuous monitoring program of the marine environment along the Fukushima coast, along with full disclosure of all information about both past and ongoing releases of contaminated water," he said.

The tests were conducted by Greenpeace monitoring teams on shore and from its Rainbow Warrior flagship, which was only allowed to test outside Japan's 20-kilometre territorial waters.

Japan has said ocean currents and tides are rapidly diluting contaminants from the tsunami-hit atomic plant, and Fukushima prefecture said that no fishing is going on at the moment in its waters.

"We have exercised self-restraint as [prefectural] safety tests have not been conducted yet," said a Fukushima official.

"We will make a decision after confirming the results of the tests, which will take place shortly."

The official added: "People do not bother fishing now. If you caught fish or other marine products in waters near the plant, they wouldn't sell."

Japan's fisheries agency, and neighbouring prefectures, have been checking marine products at different spots, and the government has prohibited fishermen from catching some species found to have elevated radiation levels.