The Japanese government said that much more needs to be done to prepare for a possible "megaquake" to reduce the feared death toll of up to 300,000 people.

Quakes are extremely hard to predict, but in January a government panel marginally increased the probability of a major jolt in the Nankai Trough off Japan in the next 30 years to 75-82 percent.

The government then released a new estimate in March saying that such a megaquake and subsequent tsunami could cause as many as 298,000 deaths and damages of up to US$2 trillion.

In 2014 the Central Disaster Management Council issued a preparedness plan recommending a series of measures that, it was hoped, would reduce deaths by 80 percent.

But the government has said that so far the steps taken would only cut the toll by 20 percent, Kyodo news agency reported, and an updated preparedness plan was issued on Tuesday.

This recommended accelerated efforts including constructing embankments and evacuation buildings as well as more regular drills to improve public readiness.

The Nankai Trough is an 800-kilometre undersea gully running parallel to Japan's Pacific coast where one tectonic plate is subducting – slowly slipping – underneath another.

Over the past 1,400 years, megaquakes in the Nankai Trough have occurred every 100 to 200 years. The last one was in 1946.

The Japan Meteorological Association in August issued its first advisory warning that the likelihood had risen but it was lifted again after a week.

Some foreign tourists have been holding off coming to Japan this summer by unfounded fears spurred on social media that a major quake is imminent.

Hong Kong-based Greater Bay Airlines reduced flights to Japan because "demand has rapidly decreased", a local tourism official said in May.

The number of visitors from Hong Kong to Japan fell 11.2 percent in May year on year, according to the tourism office. (AFP)