Since Monday, an IAEA taskforce has been in Japan to assess the country's plan to gradually release the water, which has been processed to remove most radioactive elements, into the ocean.

The organisation's deputy director general Lydie Evrard said the international team including non-IAEA experts had examined early preparations at the site for the release, expected to begin as soon as March next year.

"The IAEA taskforce made significant progress in its work this week to get a better understanding of Japan's operational and regulatory plans for the discharge of the treated water," she told reporters.

More than a million tonnes of liquid, including rain, groundwater and water used for cooling, has accumulated in tanks at the crippled Fukushima plant since it went into meltdown after a tsunami in 2011, and space is running out.

The IAEA has already endorsed the release, which it says is similar to wastewater disposal at nuclear plants elsewhere.

But neighbouring countries have expressed environmental and safety concerns, and local fishing communities are opposed, fearing it will undermine years of work to restore their reputation.

The water is treated but some radioactive elements including tritium remain. Experts say there is no evidence that would pose any danger, but opponents want the plan blocked.

Evrard said the taskforce collected water samples and gathered technical information on the trip and will release its findings in late April, the first of several reports in a multi-year review.

Ahead of the press conference on Friday, Greenpeace said it had "low expectations" for the taskforce's investigation, calling for alternative options to the release to be explored.

"The IAEA is incapable of protecting the environment, human health or human rights from radiation risks -- that's not its job," Shaun Burnie, senior nuclear specialist for Greenpeace East Asia, said in a statement.

Evrard said the UN-affiliated organisation is listening to concerns over the plans and takes them "very seriously", and the review was "aimed at providing an objective and science-based approach".