Massive downpours across southern Thailand have led to severe flooding, which has now affected around 370,000 households, or more than one million people.

The rain, which began on New Year's, has inundated thousands of villages, according to the interior ministry's disaster prevention and mitigation department.

As of Sunday, 10 of the 14 southern provinces had experienced flooding. The disaster has so far left 30 people dead with two still missing.

Over 20 main roads in southern regions have been affected, while the only major railway route leading to the south has also seen services suspended along several sections.

The mitigation department says that over 200 cases of severe flooding have been recorded in southern roads, and close to 60 bridges have suffered damage. The water remains waist deep in places.

Thailand's meteorological department on Sunday night predicted further rainy weather over the coming days and warned residents and tourists in the south to be cautious of further flooding and secondary disasters, such as landslides.

The main passage to southern Thailand, Phetkasem Road, has been at a standstill since Monday night after a bridge collapsed in a flash flood.

Thai prime minister, Prayuth Chan-ocha, on Tuesday, listed flood relief as the top priority of the government, saying they will spare no efforts to help the affected people and take more measures to accelerate the discharge operation.

The Thai navy has sent 50 discharge vessels to the two estuaries in Nakhon Si Thammarat Province, to help drain the floodwater.

It is thought that the worst of the flooding should now be over, but it will take some time for the high waters to recede.