According to a statement by the IAEA, an agency experts team advised Japan in late 2018 to take an urgent decision on the disposal of water which had been decontaminated through a process called ALPS, or advanced liquid processing system.
The IAEA said a Japanese expert committee report submitted to the government earlier this month outlined two options to dispose of the water after removing radioactive contaminants, except for tritium: vapor release and discharge into the ocean.
Mr Grossi said Japan had asked the IAEA to review the committee’s report. He described it as "comprehensive and “based on a scientific analysis".
The IAEA said it considers both disposal options technically feasible and in line with international practice.
"Once a decision is taken on the way forward, the IAEA would be ready to assist in its implementation, for example in radiation monitoring," said Mr Grossi.
"It could help provide reassurance to the public – in Japan and elsewhere – that any releases of water would be within international standards."
Earlier this month, the Japanese government held a briefing for embassy officials from nearly two dozen countries on the merits of its plan to release water from the Fukushima-Daiichi nuclear power station into the ocean.
Japan plans to remove all radioactive particles from the water except tritium, an isotope of hydrogen that is hard to separate and is considered to be relatively harmless.
"Compared to evaporation, ocean release can be done more securely," a spokesman said at the time, pointing to common practice around the world where normally operating nuclear stations release water that contains tritium into the sea.
The government is exploring ways to deal with the waste water, which already totals more than 1 million tonnes with the volume increasing by more than 100 tonnes every day.
Releasing treated water into the sea in a controlled manner is common practice at nuclear power plants and was generally considered the most viable option for Fukushima-Daichi because it could be done quickly and would cost the least.