A South Korean fishing boat, which is the object of a pending marine inquiry, sank on Tuesday night when Tropical Cyclone Jasmine hit Tonga's main island, Tongatapu.
The Shun King 8 vessel was anchored near the Nuku'alofa wharves, but broke its anchor in rough seas, overturned and sank.
The Shun King 8 lost a crew member last month when fishing in Ha'apai.
Police say they were called for assistance yesterday afternoon.
Two officers swam to the sinking vessel and helped three South Korean fishermen onto a life raft.
The pair then swam back, guiding the rescued fishermen to the shore.
Radio Tonga reports the South Koreans are well.
Several other vessels were also affected by the cyclone, including the government fishing trawler, Takuo, which sank.
Others were washed onto the shore.