Japan's nuclear watchdog has rebuked The Tokyo Electric Power Company (TEPCO) for its slack response to the leaks.
TEPCO admitted earlier this week that contaminated groundwater from the crippled Fukushima plant was leaching into the Pacific Ocean.
It has now been revealed that the plant operator knew about the problem a month ago, but despite that TEPCO continued to downplay any threat of radioactive run-off reaching the ocean.
TEPCO's inability to stem the flow of contaminated water has earned it a rare rebuke from Japan's Nuclear Regulation Authority.
About 400 tonnes of groundwater flows into the plant every day and becomes contaminated.
The head of the Nuclear Regulation Authority believes contamination of the sea has been continuing since the explosions at Fukushima following the 2011 tsunami.
TEPCO had previously failed to confirm the ground water leakage, more than two years after the worst nuclear accident since Chernobyl in 1986.
The March 2011 earthquake and tsunami knocked out cooling systems at the Fukushima plant, triggering fuel meltdowns and causing radiation leakage, food contamination and mass evacuations.
Masayuki Ono, TEPCO's general manager, told a news conference this week: "We would like to offer our deep apology for causing grave worries for many people, especially for people in Fukushima."