"The monsoon has arrived in Kerala and the large parts of adjoining Tamil Nadu," said L S Rathore, director general of the state-run India Meteorological Department.
"Conditions are favourable for further advance of monsoon into some more parts of the central Arabian Sea and Karnataka, remaining parts of Tamil Nadu, some parts of south Andhra Pradesh and some more parts of Bay of Bengal during next 48 hours," Rathore said.
According to the IMD, the monsoon has covered all of Kerala, some parts of coastal and south interior Karnataka, most parts of Tamil Nadu and some more parts of southwest and west-central Bay of Bengal.
For India, monsoon rains are crucial for farm output and many states are suffering from drought. The rains, which run from June to September, are vital for the 55 per cent of India.
Good rainfall could help the economy and keep inflation subdued, as nation prepares for a round of state polls this year and general election next year.
Last year, the monsoon hit Kerala four days after the June 1 date forecast. But overall rains in the season were just 8 percent below normal.