A police officer said the accident occurred when the was taking a number of tourists from the Hainan Association back to Songkhla province after a trip to Surat Thani for a Hainanese function.
The bus driver, who was slightly injured, told police that his passengers asked him to drive up a hill called Noen Thewada, a well-known tourist spot, for sightseeing.
However, when ascending toward the hill, the bus driver noticed that his vehicle was not powerful enough to advance uphill, therefore the bus started to slide backwards and then gathered speed.
The driver said the bus veered off the road, hit a tree and flipped, hence the casualties.
Two female passengers were crushed to death. Eighteen other passengers were injured, three of them seriously. They were rushed to nearby hospitals.
Police had already held the driver and will question the latter to determine the exact cause of the accident.