According to the one and only (good) local bird guide Gai (which means "chicken" in Thai) in Khao Sok National Park, this was his first sighting of an Asian Openbill in the area.
I found a group of at least 25 foraging in a pond and later landing in a tall tree.

Khao Sok National Park area, Surat Thani Province, Thailand, 25 January 2013