The Lake Eacham rainbowfish (Melanotaenia eachamensis) was once thought to be confined to its type locality within the Lake Eacham World Heritage National Park. M. eachamensis disappeared from the lake following the translocation of several species into the lake and the species was pronounced extinct in the wild in 1987. In a 2007 survey we noticed that rainbowfish were present in the lake once again. We used a molecular marker to identify these fish and the likely source population. Analysis of the D-loop region of mitochondrial DNA revealed that the species now present in the lake is Melanotaenia splendida, and is most closely related to several M. splendida populations in the immediate vicinity. Here we explore a range of scenarios that may have led to this colonisation event and highlight the dangers associated with translocation.