Box jellyfish
In Australia (mainly northern; however, box jellyfish can live in subtropical waters as far south as Sydney, with "Carybdea marsupialis" being recorded as far south as Bermagui, Sapphire Coast) the highest risk period for the box jellyfish is between October and May, but stings and specimens have been reported all months of the year. Similarly, the highest risk conditions are those with calm water and a light, onshore breeze; however, stings and specimens have been reported in all conditions.
Irukandji jellyfish
Irukandji jellyfish ( ) are any of several similar, extremely venomous species of box jellyfish. With an adult size of roughly a cubic centimetre (1 cm), they are both the smallest and one of the most venomous jellyfish in the world. They inhabit the northern marine waters of Australia. They are able to fire their stingers into their victim, causing symptoms collectively known as Irukandji syndrome. There are about 16 known species of Irukandji, of which "Carukia barnesi", "Malo kingi", "Malo maxima", "Malo filipina" and "Malo bella" are the best-known.