A 35-metre (115-foot) tsunami could hit the Japanese coast in the wake of a massive earthquake, an expert panel has said after revising its worst case scenario projections following last year's disaster.

If a 9.0-magnitude quake struck in the Nankai Trough off central to western Japan huge swathes of the Pacific coastline could be inundated, with 20-metre-plus waves hitting areas from Tokyo down to the southwestern island of Kyushu.

At the town of Kuroshio in southwestern Kochi prefecture the tsunami could reach 34.4 metres -- the highest level projected under the scenario, the Cabinet Office panel said late Saturday.

And at the now offline Hamaoka nuclear plant in central Shizuoka prefecture, the tsunami could be as high as 21 metres, breaching the 18-metre breakwater that operators are currently constructing, the panel said.

In its previous projection in 2003 the panel gave a worst case scenario in which no areas would be hit by a tsunami of more than 20 metres.

But the panel has upgraded its predictions in the wake of the 9.0-magnitude earthquake on March 11 last year that sent a tsunami barrelling into the northeast, killing some 19,000 people and devastating the coastline.

Waves of more than 15 metres hit cities including Ishinomaki, Soma, and Ofunato.

The monster tsunami crippled cooling systems at the Fukushima nuclear power plant, sparking a crisis which is still ongoing.

The panel noted that the projection was for "the worst possible tsunami" and the "chance of its occurrence is extremely low".

It said it will continue studying the extent of areas that could be hit and submerged by a tsunami, while the government will examine its emergency disaster measures based on the latest estimation.