The Swain Reefs is an area of 2000 km² of reefs and small coral cays at the southern end of the Great Barrier Reef in Queensland, Australia. Up to 5000 non-breeding Roseate Terns (Sterna dougallii) have been observed in the area during July, and 25 000 in January or February. We used a small cannon net to capture birds that we then banded and leg-flagged; we also collected biometric and moult data from appropriate samples. We captured 368 Roseate Terns in July of 1999, 2000 and 2001, and 3044 in January of 2002 and 2003. We determined that at least some Roseate Terns of the local Queensland breeding population (S. d. gracilis) from colonies on the Capricornia Cays occur in the Swain Reefs during July. Among the January captures were 44 birds carrying Japanese leg-bands and three carrying Taiwanese leg-bands. This is the first evidence that Asian-breeding Roseate Terns over-winter in the southern hemisphere. Subsequent sightings of flagged birds have occurred in the Japanese breeding colonies (with one recapture) and in breeding colonies in the southern Great Barrier Reef. Using plumage and moult characteristics we determined that 60% (15 000 birds) of the Roseate Terns present in Swain Reefs in January belong to the Asian population (S. d. bangsi), 1.5% (375 birds) to the local population (S. d. gracilis), and the remaining 38% (9500 birds) were of unknown breeding origin.