Arriving with rescue workers at the scene in front of the Baan Thai Beach Resort on Thaweewong Rd at 2pm, police found Dwayne David House, 23, lying on his back on the road with a stab wound below the left side of his rib.
A friend, also Australian, called on officers to arrest three Thai boys standing nearby, which they did.
After questioning the two Australians, police arrested one of the Thais, a 15-year-old, who was identified as having done the damage, and released the other two.
Mr House was taken to Patong Hospital for treatment.