Gononemertes australiensis is a parasitic ribbon worm. It lives commensally in the ascidian Pyura pachydermatina (found in the sublittoral waters of the Sydney region of New South Wales), specifically in its atrium.It is dioecious and has several gonads. Each of its gonads produce several oocytes while the male worms carry testes along its parenchyma. Fertilization is external.