At least six people have died and five others are wounded in eastern Japan as record rainfall pummelled the Chiba prefecture near the capital Tokyo.

The rains, which authorities have described as "unprecedented", cut power to more than 20,000 households while 7,000 travellers were stranded overnight at Tokyo's Narita airport.

Local authorities issued a level five emergency warning, its highest alert, on Thursday. This was downgraded to a level four by Friday morning, but they continue to warn of landslide and flooding risks.

The deluge comes after a series of storms swept through Japan and neighbouring countries this week, in what's been forecast to be an unusually active typhoon season.

Japan's Meteorological Agency said the most recent deluge was brought by warm, humid air flowing into the country's east that combined with cold air above to make atmospheric conditions extremely unstable.

The rains are expected to last through late on Friday.

"This case was an extremely unusual situation, even by Japan's historical weather standards," Chiba Governor Toshihito Kumagai told reporters on Friday morning.

"I have responded to many disasters in the past, but I have never experienced a case like this."

Chiba, which sits right next to the capital city of Tokyo, has logged more than 100mm of rain per hour since Thursday afternoon, authorities say.

This is about three times the average rainfall in August, and the highest volume of precipitation ever recorded in the country.

Authorities would prioritise saving lives and persist with rescue efforts, Kumagai added.

At lest six people have died, according to reports from public broadcaster NHK, Kyodo news agency and Asahi newspaper.

Among those killed was a woman who died inside her car submerged on a flooded road, authorities said. Two others collapsed along the streets and later died.

The Japan Meteorological Agency earlier described the situation as an "unprecedented level of heavy rain".

Several rail services remain suspended on Friday morning while some major highways are also shut.

More than 100,000 households have been asked to evacuate on Thursday after the rain triggered landslides and power outages.

Thousands were stranded overnight at Narita as the storm disrupted transportation to and from the airport

All flights are scheduled to operate normally on Friday, a spokesperson from Narita airport said. Flag carrier Japan Airlines said some of its flights may be delayed but that it does not expect any cancellations.

Passengers were stranded at metro stations, while around 1,800 people spent the night at the Chiba Prefectural Government building.

A woman in Chiba's Ichikawa city told NHK the rain "happened all of a sudden so it was a complete surprise".

"The power is out so I'm worried whether I can open my business in the next couple of days," she said.

Meanwhile in the Philippines, authorities have again shut schools and government offices are again shut in most of the country's north, citing the threat of floods.

Earlier this week, Japan, the Philippines and China were hit by back-to-back storms – Dolphin, Chan-Hom and Peilou – churning across the region at the same time.

Dolphin brought heavy rain to Japan's Okinawa island, cutting power to tens of thousands of buildings, toppling trees and cancelling flights.

Over the weekend, Chinese authorities evacuated more than a million people from their homes as Dolphin, the strongest typhoon to hit China so far this year, made landfall in the country's east.

Additional reporting by Julie Masuda