The official death toll from months of flooding in Thailand rose above 800 on Saturday, as the country's king said the damage caused by the waters was the "worst ever".

Late reporting of deaths from the inundations in central and northern provinces -- now largely receded -- have seen fatality numbers continue to rise, the interior ministry said.

A total of 823 flood-related deaths have now been recorded, including 10 in heavy November monsoon rains in the south of the kingdom, since the crisis began in July. Three people are still missing.

In his annual New Year speech, King Bhumibol Adulyadej regretted the devastation caused by the deluge and sent his sympathy to those affected.

"The damage from the flood is the worst ever. It will serve as a reminder to people to be prepared because life is not always happy and people sometimes have to face hardship," he said.

At their height Thailand's worst floods in half a century affected 65 of 77 provinces in the low-lying nation, deluged hundreds of thousands of homes and forced the closure of large industrial parks, disrupting global supply chains.

The waters have since receded significantly, but more than two million people in five provinces continue to be affected and many areas still face a major clean-up operation.