Mesopelagic fishes and crustaceans inside a newly formed, warm-core eddy off eastern Australia were similar in composition and abundance of species to those from the more northerly Coral Sea and East Australian Current (EAC) source waters of the eddy. The crustaceans inside the eddy were a mixture of warm-water and cold-water species and were dissimilar to the crustacean fauna on either the Tasman Sea or the Coral Sea-EAC sampling sites. In contrast, of the fishes collected (all from the family Myctophidae), only warm-water or widespread species were present in the eddy. A zoogeographic transect along 155° between 16°S and 32°S showed cold-water crustaceans were present between 24°S and 32°S whereas cold-water fishes were absent. The crustacean fauna of a new eddy was distinct from both the fauna of the source region and of the Tasman Sea because sufficient cold-water species were already present in the geographical location where a new eddy forms. The myctophid fauna of a new warm-core eddy was essentially identical to the Coral Sea fauna because cold-water species do not penetrate into the warmer waters of the source of the eddies. A temperature of 15°C at 250 m depth may be a useful oceanographic marker separating subtropical from temperate myctophids in the Coral and Tasman seas.