The release of wastewater from Japan's stricken Fukushima nuclear plant into the Pacific will begin on Thursday, Prime Minister Fumio Kishida has announced, despite opposition from fishermen and protests by Beijing.

Japan insists the gradual release of the more than 500 Olympic swimming pools' worth of water is safe, a view backed by the UN atomic agency.

"We will request Tepco to promptly prepare for the start of oceanic discharge based on the plan approved by the Nuclear Regulation Authority, with discharge expected to be August 24 if weather and sea conditions do not hinder it," Kishida said on Tuesday following a ministers' meeting in Tokyo, referring to the stricken plant's operator.

In one of the world's worst-ever atomic disasters, the Fukushima-Daiichi nuclear plant was knocked out by a massive earthquake and tsunami that killed around 18,000 people in March 2011.

Since then, operator Tepco has collected 1.34 million tonnes of water used to cool what remains of the still highly radioactive reactors, mixed with groundwater and rain that has seeped in.

Tepco says that the water has been diluted and filtered to remove all radioactive substances except tritium, levels of which are far below dangerous levels.

This water will now be released into the ocean at a maximum rate of 500,000 litres per day out off Japan's northeast coast.

China has accused Japan of treating the ocean like a "sewer", banning imports of food from 10 Japanese prefectures even before the release and imposing strict radiation checks. (AFP)