At least 100 people have died in northern India over the last three days in unusually heavy late monsoon rains which have submerged streets, hospital wards and houses, officials said on Monday.

Dozens of boats were pressed into service on streets overflowing with gushing rain water in Patna, the capital of the eastern state of Bihar, after torrential downpours far stronger the normal.

At least 27 people have lost their lives across the state and another 63 in neighbouring Uttar Pradesh since Friday, authorities said. With more rain predicted, weather experts say September could end as the wettest in more than a hundred years.

"Patna alone has recorded some 226 millimetres (8.9 inches) of rainfall since Friday," Bihar disaster response official M. Ramachandru told AFP.

Photos showed patients lying on hospital beds in dirty rain water at the state-run Nalanda Medical College and Hospital in Patna.

It has also been raining heavily in southern India and in the western state of Gujarat.

The annual monsoon usually lasts from June to September.

With the Indian Meteorological Department on Monday predicting excess rainfall across 15 states, this year's monsoon will end as the wettest since 1917, the mass-circulation Times of India said.

"There are no signs of withdrawal for at least four-five days," senior IMD officer Mrutyunjay Mohapatra told the daily.

The monsoon, which is vital for farmers across the South Asian region, killed some 650 people in India, Nepal, Bangladesh and Pakistan in July this year.