A fresh leak of radioactive water into the open ocean has been discovered at Japan's crippled Fukushima nuclear complex, its operator said Monday as cleanup efforts continued.

Crews were dispatched to find the cause of the runoff, which was discovered Sunday near a system used to treat contaminated water, according to Tokyo Electric Power Co. (TEPCO).

The plant operator said it was using sand bags to prevent further leakage after 45 tonnes of water were found pooled around the water-treatment system's condensation unit.

Most of the water remained inside the building, but about 300 litres (79 US gallons) was estimated to have escaped and run into a nearby gutter that leads to the ocean before crews could contain the leaks, a company spokesman said.

The water was contaminated with the radioactive substances caesium and iodine but their levels were "roughly the same as or slightly higher" than ocean water near the plant, the spokesman said.

The water may also contain other harmful substances, such as strontium, the company said. Strontium is linked to bone cancers.

It may take two to three weeks before it can be determined how much, if any, strontium is contained in the water, the utility said.

"We will keep investigating the problem" with the system, the spokesman said.

In the weeks after the March 11 earthquake and tsunami hit the plant, TEPCO dumped 10,000 tonnes of lower-level radioactive water into the Pacific Ocean.

Subsequent reports have found the radiation was widely dispersed and did not pose a threat to human or animal life.

Fukushima's makeshift water-treatment system has been hit by a series of problems which forced officials to temporarily shut it down.

But TEPCO said the leak would not hinder its plans to bring the reactors to a state of cold shutdown by the end of the year.

Large areas around the Fukushima plant have been left contaminated with radiation after a series of meltdowns in the world's worst nuclear accident since Chernobyl.

The accident has not directly claimed any lives, but has left tens of thousands of people displaced and rendered whole towns uninhabitable, possibly for decades.