Overland routes to Thailand's flood-hit south were cut on Tuesday after two bridges collapsed following days of torrential rain that have killed at least 30 people, including a five-year-old girl.

The heaviest January rains for three decades have lashed the country's south for more than a week, affecting 1.1 million people across twelve provinces.

The unseasonal downpours have also put a dampener on Thailand's peak tourist season, prompting cancellations on popular resort islands including Samui and Phangan.

The Highways Department said the main road heading down Thailand's southern neck was closed after two bridges collapsed in Prachuab Kiri Khan province.

Trains south have also been halted by the rising floodwaters, increasing demand on already stretched flights to and from the flood-ravaged region.

The death toll has crept up in recent days as floods have reached rooftop level in some areas.

A five-year-old girl became one of the latest victims when a flash flood hit a van she was travelling in late Monday in Prachuab Kiri Khan province.

"Her family climbed to the roof of the van to avoid the water but she fell in with her mother," relief worker Rawiroj Thammee told AFP.

"The girl was swept away... villagers found her body 200 metres from the van this morning."

- 'Lost everything' -

January usually sees visitors flocking to southern Thailand's pristine beaches as monsoon rains abate and temperatures ease.

But the region has been battered by what the ruling junta describes as the heaviest January rainfall in 30 years.

Vast tracts of the south -- an agricultural hub for rubber, palm oil and fruit plantations -- have been left under water while flash floods have caused deaths and widespread damage.

Soldiers have been delivering relief packages by helicopter and boat to those stranded in the worst-hit areas.

In Surat Thani province, a tourist gateway to the party islands of Samui and Phangan, villagers said a week of rain had brought an unprecedented deluge.

"Every year it floods, but not like this," Chamnan Ingkaew, a village leader in Chaiya district, told AFP.

"There are 100 houses in my village, but we all had to leave and everything inside was lost... the water kept coming and coming, almost two metres high."

Prime Minister Prayut Chan-O-Cha on Tuesday said residents should have heeded evacuation warnings issued before the floods.

"Many people do not want to leave, they want to stay home," he said, adding their reluctance was making the relief effort more pressing.

Prayut, who also heads the junta, said the unbridled growth of towns and cities without planning for drainage was making Thailand increasingly vulnerable to floods.

Local media have shown images of asphalt roads cleaved into pieces, uprooted trees and piles of debris in places where the waters have receded.

In remote flooded hamlets, villagers have been forced to wade through muddy waters with a few salvaged belongings held above their heads.

Patients were evacuated by canoes as a hospital was swamped with waters in Prachuab Kiri Khan.

The rain is forecast to slacken over the next 24 hours in some areas.