Toxic radioactive substances have once again been detected in groundwater at the crippled Fukushima nuclear plant, its Japanese operator says, the latest in a series of incidents at the tsunami-battered complex.

Tokyo Electric Power Co (TEPCO) said on Sunday that tests showed that tritium, a radioactive isotope of hydrogen used in glow-in-the-dark watches, was present at levels 10 times the permitted rate.

Still a threat: Bone cancer causing substance detected at Japan's Fukushima Dai-ichi nuclear plant. Credit:AP

"From test samples on July 5 ... we detected a record high 600,000 becquerels per litre" of tritium, 10 times higher than the government guideline of 60,000 becquerels per litre, TEPCO said in a statement.

"We continue efforts to prevent further expansion of contamination by construction works ... and will strengthen monitoring of pollution comprehensively," it said.