Power has been restored to two fuel storage pools at Japan's tsunami-damaged nuclear plant, but two others have been without fresh cooling water for more than a day.

Tokyo Electric Power said that pool temperatures at the Fukushima Dai-ichi plant were well within safe levels, and that pools would remain safe for at least four days without fresh cooling water. The utility said the reactors were unaffected and no other abnormalities were found.

The cooling system was restored at two of the four pools and the systems for the two other pools were poised to resume by Wednesday morning as workers complete repairs and try to determine the cause of the problem.

About 50 workers in protective suits and full-face masks were mobilised to fix cabling that involved the last of the three switchboards that they suspect as a possible cause of the problem. The utility was also preparing a backup system in case the repairs did not fix the issue and "the worse comes to worst," Ono said.

Japan's 2011 earthquake and tsunami destroyed the plant's power and cooling systems, causing three reactor cores to melt and fuel storage pools to overheat. Massive radiation leaks from the plant have contaminated air, water and soil around the plant, causing some 160,000 residents to evacuate.

The current power outage is a test for TEPCO to show if it has learned anything from the disaster. TEPCO, which has repeatedly faced cover-up scandals, was slammed by local media Tuesday for waiting hours to disclose the blackout.