John Huisman from the Western Australian Herbarium has pointed out that this Taxonomy Australia blog has an error. In a eulogy to the Derwent River sea star, Tim O'Hara from Museums Victoria described its loss as the first known marine extinction in Australia.
In fact, as this article points out, the spectacular and beautiful marine red seaweed Vanvoorstia bennettiana also appears to be extinct. Only known from two locations in Sydney Harbour, Vanvoorstia bennettiana has not been seen since at least 1913, despite extensive searches and surveys in its own known habitat.
It appears to have been particularly susceptible to disturbances such as siltation following dredging, and in addition may have been over-collected due to its beauty and taxonomic interest to phycologists and seaweed enthusiasts.
Of course, there are likely to be many 'invisible extinctions' in the marine realm, given that it is understudied compared with terrestrial habitats. This is one more reason why marine taxonomy and documentation of marine biodiversity is so important.