Authorities in Thailand plan to send helicopters on Wednesday to evacuate critically-ill patients from a southern hospital marooned by some of the region's worst floods in years, as the death toll rose to 33, with more rain expected.

Floods have swept through nine Thai provinces and eight states in neighbouring Malaysia for a second successive year, prompting both countries to evacuate nearly 45,000 people.

In Indonesia, eight to 13 people are estimated dead following floods and landslides this week, while one has died in Malaysia.

In Thailand's hardest-hit city of Hat Yai, a public health official said helicopters would deliver food and ferry out patients after the first floor of the main government hospital treating 600, some 50 of them in intensive care, was inundated.

"Today, all intensive care patients will be transported out of Hat Yai Hospital," the ministry official, Somrerk Chungsaman, told Reuters.

About 20 helicopters and 200 boats drafted into the Hat Yai rescue effort have had difficulty reaching stranded people, government spokesman Siripong Angkasakulkiat told reporters.

Boats can carry in supplies when waters recede

Patients, relatives and medical staff at the hospital number around 2,000 and boats should be able to carry in food as the waters recede, Somrerk said.

On a single day last week Hat Yai received 335 mm (13 inches) of rain, for its highest such tally in 300 years.

Military helicopters were also carrying generators to the hospital, the Thai Navy said, posting photographs on social media of equipment being moved to a rooftop under dark grey skies.

Floods across nine Thai provinces, including Songkhla, where Hat Yai is located, have affected more than 980,000 homes and over 2.7 million people, the interior ministry said.

Thai weather officials forecast scattered thundershowers and heavy rains on Wednesday in several southern provinces, including Songkhla.

Convoys of aircraft and trucks were moving flat-bottomed boats and rubber dinghies towards Hat Yai, along with medical supplies and personnel, said the Thai military, which took charge of relief efforts on Tuesday.

Sole Thai aircraft carrier joins rescue

Thailand's only aircraft carrier, Chakri Naruebet, set out from its home port on Tuesday to provide air support, medical assistance and meals in the relief efforts, the navy said.

Rescuers pulled stranded families, including children and the elderly, from homes inundated by swirling brown waters, photographs posted by the Thai army showed.

Many of the stranded took to websites and social media to seek help.

"Please help. I'm very worried about my mother," wrote one person, adding that she had been unable to contact the 53-year-old in Hat Yai since Saturday, when domestic supplies were down to just a bottle of water and two packs of instant noodles.