Surat Thani governor Auychai Inthanak said on Monday that the two islands were among the 16 districts declared disaster-hit areas of all 19 districts in the province as they were damaged by floods and heavy rain.
Surat Thani is one of the worst-hit provinces in the region, which has been blanketed by downpours since last week.
The Meteorological Department on Monday warned of severe weather in the region.
Flooding already forced the State Railway of Thailand to cancel train services southward of Thung Song junction station in Thung Song district, Nakhon Si Thammarat province, at least until Tuesday.
Flooding on Koh Samui eased on Sunday but downpours overnight left roads inundated again on Monday.
The situation looks worse due to overflowing Phru Chawaeng, the island’s main reservoir, into low-lying communities on the island.
The weather office predicted rain in most areas on Koh Phangan until Wednesday.
On Koh Phangan, a road between Ban Tai and Tong Nai Pan, a main route for tourists on the island, was impassable after fallen mountain rocks brought down four electricity poles, causing blackouts in some areas.
Workers from the Provincial Electricity Authority later cleared the blockade and were trying to repair the damaged poles.