Melanotaenia australis can grow to a length of around 10 cm, but are more common at 8 cm or less. Males are usually much larger and deeper bodied than females. Their colouration, particularly the fins and intensity of the mid-lateral stripe, is highly variable depending on their geographic locality. This is mainly due to a rapid speciation in the rainbowfish group and is further enhanced by the degree of random morphological variation occurring among the various populations. Colouration generally consists of 1-2 broad, dark mid-lateral stripes and a series of narrow longitudinal stripes of orange-red corresponding with each scale row. The whole body has an overall sheen of iridescent purple or blue. Fins range from nearly colourless to deep red, or clear with red or green flecks. The second dorsal and anal fins have reddish orange rays with a yellow membrane and dark distal border. Gerry Allen notes that the real trademark of this species is the zigzagging blackish lines on the lower sides, just above the anal fins. The black mid-lateral line may be very prominent or scarcely apparent.(Melanotaenia australis)