Lt Paratpong Kaewgaemjan, head of the bomb squad, ordered the area cordoned off before examining the devices, which were identified as M26A2 grenades, still in good condition.
Burmese worker Nay A told police that he was sorting out rubbish before burning it, when he came across the plastic bag with the grenades in it.
“I lifted the bag and took a peek inside. When I saw the grenades I put the bag down where I found it and told my boss,” he said.
Police said they believe the grenades either belonged to someone who wanted to get rid of them before they were found in possession of illegal arms, or had been stashed at the dump by teenage gang members for later use against rivals.
Either way, they said, they will identify the owner.
This is the second time in six weeks that grenades have been found apparently dumped in Phuket. On October 28 an M67 grenade was found on the beach at Cape Panwa.