The heaviest rains in 30 years have caused the floods sweeping southern Thailand, the kingdom's junta leader said Monday, as the army ramped up efforts to reach remote inundated areas.

At least 21 people have been killed and nearly one million people affected by the floods after days of unseasonably heavy rain across the country's southern neck.

Downpours and flash floods have also disrupted holidays on tourist islands including Samui and Phangan, disappointing tens of thousands of visitors hunting Thailand's peak season sun.

Despite the end of the monsoon season weeks ago, there were no immediate signs of respite for the flood-battered region.

The "severe flooding is because of the worst rainfall in more than 30 years", Prime Minister Prayut Chan-O-Cha told reporters.

Prayut, who led the 2014 coup, warned climate change meant floods and droughts were increasingly likely and "Thailand must prepare to handle these problems".

Soldiers have been deployed to provide relief packages and rescue stranded people in the worst-hit areas.

Meanwhile boats with special pumping equipment have been dispatched to Nakhon Si Thammarat province, which has seen floods reach rooftops in some areas, causing deaths and extensive damage to property.

Television images showed villagers wading through muddy water in remote flooded hamlets, scant belongings held high.

In areas where the water had receded, locals slopped out mud from their homes and surveyed broken roads, bridges and ruined farmland.

Thailand's south is heavily reliant on tourism and agriculture, including rubber, fruit and palm oil plantations, and the floods will likely have a significant economic impact.

In the northeast -- a prominent rice growing region -- farmers face a different dilemma, a water shortage caused by two years of drought.

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