Biodiversity offsets are increasingly used in policy frameworks to regulate the environmental impacts of development including projects located in marine environments. Scientific knowledge gaps and other practical challenges have necessitated flexibility concerning the manner in which key offsetting principles are implemented in policy frameworks relevant to such environments. The potential trade-off of such flexibility is that consequent marine offsetting practice may not be compatible with the ultimate objective of no net loss of biodiversity. Here we present a systematic review of marine and coastal development projects in Australia, examining how offsetting is being implemented in practice. Forty-two (42) projects were assessed, predominantly located in Queensland and Western Australia and associated with the development and operation of ports. We find that application of key biodiversity offsetting principles (e.g. ecological equivalence) was frequently incomplete or absent. For approximately 50% of reviewed projects we were unable to identify public information concerning how offsetting requirements were established. The current environmental outcomes of marine biodiversity offsetting in Australia are unclear but there are indications that it is unlikely to achieve no net loss of biodiversity.