During the interglacial periods the rising sea cleared the coastal vegetation, eroded the soil and created large dune systems.  The heavier sediments were carried on shore to form the new dunes.

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Multiple dunes were formed on the coasts and across former valleys lthere were large sand islands, called Barriers. These enclosed lagoons such as the Tuggerah Lakes and helped to enclose Lake Macquarie. The later is the largest tidal lagoon in the Southern Hemiphere.