The water would have receded by then, exposing large parts of the seabed and leaving behind what I fondly call "islands", as a result of the uneven elevations of the seabed. These exposed portions are called sandbars, as pointed out by Daniel Dendy (see comment).
These islands serve as "refuge" for seabirds and playgrounds for the beachgoers. Even adults play out their childhood fantasies on these islands.
This "Insea Land" phenomenon is a feature of the Altona part of Port Phillip Bay. If you have not been here before, come and see the islands, watch the setting sun and enjoy the evening breeze from the Pier.