© 2016 by the authors.In contrast to many tropical animals expanding southwards on the Australian coast concomitant with climate change, here we report a temperate endemic newly found in the tropics. Chromodorid nudibranchs are bright, colourful animals that rarely go unnoticed by divers and underwater photographers. The discovery of a new population, with divergent colouration is therefore significant. DNA sequencing confirms that despite departures from the known phenotypic variation, the specimen represents northern Goniobranchus splendidus and not an unknown close relative. Goniobranchus tinctorius represents the sister taxa to G. splendidus. With regard to secondary defences, the oxygenated terpenes found previously in this specimen are partially unique but also overlap with other G. splendidus from southern Queensland (QLD) and New South Wales (NSW). The tropical specimen from Mackay contains extracapsular yolk like other G. splendidus. This previously unknown tropical population may contribute selectively advantageous genes to cold-water species threatened by climate change. Competitive exclusion may explain why G. splendidus does not strongly overlap with its widespread sister taxon.