The early life stages of Hormosira banksii can be viewed as a series of distinct developmental events such as gamete release, fertilization, germination, adhesion, cell division, apical hair formation and growth. These developmental stages can be targeted for experimentation as well as used as a measure of health in bioassay tests. Three bioassays were devised and compared to determine which developmental events in H. banksii are most susceptible to effluent (40%) and/or reduced salinity (21‰ S). These treatments approximate conditions in the shore zone where H. banksii once occurred at Boags Rocks, Victoria, Australia. Reduced salinity and sewage effluent affected developing H. banksii differently. Damage caused by reduced salinity occurred within the first 48 h and affected fertilization and germination. Exposure to sewage effluent did not affect fertilization but reduced the number of healthy embryos that had formed a wall by 24 h, germinated by 48 h and divided by 72 h. Both sewage effluent (40%) and reduced salinity (21‰ S) significantly decreased the number of healthy developing embryos that had adhered by 24 and 48 h.