Three cruises surveying the eastern Australian shelf revealed the occurrence of coralline algal buildups at depths between 80 and 120 m in tropical (Capricorn area) and subtropical (off Fraser Island) settings. The buildups, decimeters to several meters high, started to grow on an erosion surface during the Holocene transgression in water depths within ∼ 30 m of their present-day depth, and continued to present, as indicated by living covers of coralline algae. The buildup framework is a boundstone of encrusting coralline algae growing one over the other. The result is an open structure partially filled by mudstone to packstone internal sediment and minor marine cements, and affected by several phases of bioerosion. Mesophyllum is the main algal builder. Other melobesioids and Sporolithon appear in most samples. The tropical Capricorn buildups comprise algal assemblages slightly more diverse than the ones in the subtropical examples off Fraser Island. The buildup accretion involves many phases of framework growth, bioerosion, and sediment infilling at low average rates (maximum 2 to 3 cm/k.y.). These modern examples demonstrate that deep-water algal mounds can be coeval with shallow-water coral reefs and can be found in outer-platform and platform-edge deposits in ancient tropical platforms (e.g., Huon Gulf, Papua New Guinea). Upper Paleozoic phylloid algal mounds built by an open framework of Archaeolithophyllum crusts are similar to the northeastern Australian Mesophyllum-dominated boundstones, indicating they may have developed in similar deep-water, open-platform settings.