The International Atomic Energy Agency (IAEA) head will meet Japan's prime minister and foreign minister in Tokyo on Tuesday before heading to Fukushima on Wednesday to visit the devastated plant.

Several of the nuclear facility's reactors went into meltdown after cooling systems were overwhelmed by a massive 2011 tsunami.

The resulting nuclear accident was the worst since Chernobyl, and the clean-up has lasted more than a decade, with most areas declared off-limits due to radiation now reopened.

Decommissioning the plant itself will take decades, but the facility's operator TEPCO faces the immediate problem of more than 1.33 million cubic metres of water accumulated on the site.

The water is a mixture of groundwater, rain that seeps into the area, and water used for cooling.

It is processed through a facility that TEPCO says removes almost all radionuclides but tritium, which commonly remains in waste water discharged into the sea by nuclear plants globally.

Japan plans to dilute the treated water and release it over decades via a pipe extending around a kilometre from the eastern coast where the plant is located.

The proposal has already been endorsed by the IAEA, but the government has said the release will only begin after a "comprehensive review" that Grossi will present Tuesday.

"A review by IAEA, given how authoritative it is in the management and application of nuclear safety standards, is critical to our efforts to foster international understanding," top government spokesman Hirokazu Matsuno said Tuesday.

Still, the release remains controversial, with Beijing vocally criticising the plans, and some in South Korea panic-buying salt over fears of contamination after the discharge begins.

Fishing communities in Fukushima are also worried customers will shun their catches, despite strict testing protocols for food from the region.

Japan has said the release will start this summer, without giving further details, and Matsuno said that was still the plan.

"We will thoroughly explain and communicate, both domestically and internationally, details of the IAEA report, our effort to ensure safety and measures against reputational damage," he added.