The sulphonium compound diinethylsulphoniopropionate (DMSP) is commonly found in temperate green macroalgae. To examine taxonomic and regional and local geographical patterns of DMSP production in Australasian algae, I collected 30 species of green algae from 14 sites in three regions, eastern Australia, Tasmania, Australia, and the North Island of New Zealand. The distribution of DMSP content was similar to that seen from other areas of the world. DMSP was found in high concentrations in Ulva and Codium spp. It tended to be undetectable or in lower concentrations in other members of the orders Bryopsidales and Cladophorales. There was no evidence for differences in concentrations among the three regions in the genera Codium and Ulva; however, the invasive subspecies of Codium fragile, C. fragile ssp. tomentosoides, had significantly higher concentrations of DMSP than the non-invasive subspecies. The herbivorous sea slug Elysia maoria had whole body concentrations that were not significantly different from those of its host alga C. fragile ssp. tomentosoides. The distribution patterns of DMSP in Codium spp. do not support the hypothesis that DMSP is used as an antioxidant in this genus. Based on the data collected here and previous reports from the literature, I speculate that one function of DMSP in these algae may be to deter herbivores.