Tokyo Electric Power (Tepco) staff also failed to keep records of storage tank inspections, said Nuclear Regulation Authority commissioner Toyoshi Fuketa.
Tepco admitted on Tuesday that at least 300 tonnes of the water had leaked into the soil from one of the tanks.
Mr Fuketa upped pressure on Tepco after a visit to the plant.
"Fundamentally, for a facility holding that kind of radioactive water, they did not take action that foresaw the risk of possible leaks," he said.
Authority chairman Shunichi Tanaka said he was concerned that others among the over 300 hastily built tanks would fail.
But experts say water is already leaking all over the site.