Japanese rescuers completed building a slope on Saturday to reach a truck driver stuck in a sinkhole, officials said, four days after his vehicle was swallowed by the cavity now 40 metres wide.

Rescuers have been struggling with the operation as the hole – now the length of an Olympic swimming pool – expands since opening up on Tuesday in Yashio city, on the outskirts of Tokyo.

"We have just completed the work on the ramp," Saitama regional governor Motohiro Ono told reporters.

Rescuers will use the 30-metre slope to send heavy equipment into the hole.

"We are going to clear the debris and rescue the driver as soon as possible," Ono said.

Soil and debris covering the cabin of the 74-year-old driver have prevented any communication with him since midday on Tuesday.

Groundwater leaking into the sinkhole was mostly stopped on Saturday, public broadcaster NHK reported.

With the walls of the hole – around 15 metres deep, according to NHK – eroding, rescue workers are unable to stay inside it for long.

The hole was initially around five metres in diameter but combined with a larger cavity that opened during the rescue operation on Tuesday night.

Heavy chunks of asphalt have fallen inside, making it difficult for rescue workers and heavy machinery to go near the chasm.

The 1.2 million people living in the area have been asked to cut back on showers and laundry to prevent leaking sewage from hindering the operation.

Around 2,600 cases of road sinkholes in 2022 were caused by sewer pipes, according to local media. Most were small, at only 50 centimetres deep or less.

In 2016, a giant sinkhole around 30 metres wide and 15 metres deep appeared on a busy street in Fukuoka city, triggered by nearby subway construction.

No one was hurt and the street reopened a week after workers toiled around the clock. (AFP)