Annual monsoon rains arrived in southern India on Wednesday, easing fears of millions of desperate farmers after two straight years of drought, the weather department said.

Farmers rely on the monsoon rains, which hit the Kerala coast every year and then sweep across the country, to water their crops and replenish dams and reservoirs.

But India is in the grip of its worst water crisis in years, with 330 million people, or a quarter of the population, suffering from drought after two weak rainy seasons.

"Southwest monsoon hit Kerala today," the Indian Meteorological Department tweeted, adding that conditions were right for the rains to advance across the rest of southern India.

The department had earlier forecast an above-average monsoon this year and a start date of June 7, offering hope for the struggling agricultural sector.

About 60 percent of the workforce is employed in the sector, mainly as poor labourers. Farmers plant many of their crops from June with harvesting from October.

The arrival of the monsoon is always keenly watched in India, but even more so this year as much of the country reels from the drought.

Drinking water is running short in many states and poor rains have prompted extreme measures, including stationing armed guards at reservoirs and sending water trains to the worst-affected regions.