Tokyo Electric Power Co (TEPCO) said that high levels of radioactivity were detected in samples taken Friday from the seabed, in places 20-30 metres deep, Kyodo news agency reported.
The plant was damaged by a magnitude-9 earthquake and ensuing tsunami on March 11 and has been leaking radioactive material ever since.
A monitoring system at the Fukushima plant failed in the power outage that took out the plant's cooling systems after the March disaster, depriving authorities of vital information to map radiation contamination, Kyodo reported Tuesday.
The loss of the data feed from the Emergency Response Support System (ERSS) probably delayed the evacuation order around the plant, about 250 kilometres north-east of Tokyo, the report said.
The government was criticized last week for not mapping the potential contamination of the area fast enough, and when it started to do so as late as April, not releasing the information to the public.
About 5,000 data sets were released Tuesday on the website of the Nuclear Safety Commission, mapping the spread of contamination at hourly intervals from early April.
The failure of the ERSS and another impact assessment system, which cost 28 billion yen (345 million dollars) to install and maintain, has raised questions about the disaster readiness of Japan's nuclear sector.
Meanwhile, shareholders of Japan's five electric power companies urged them to close their atomic power stations in the wake of the nuclear crisis.
Some 400 TEPCO shareholders submitted official documents in support of the proposal, public broadcaster NHK reported.
The report did not say whether the petitioners were among TEPCO's approximately 4,500 institutional shareholders, or its more than 596,000 private investors.
Stockholders of at least four other major utilities have made similar proposals, NHK said. The concerned investors said the risks of nuclear power generation were too big for a single company to handle. TEPCO has also been asked by its shareholders to stop investing in a fuel reprocessing plant northern Japan, Jiji Press reported.