Sydney Harbour's present form was created by changes in sea level which occurred over many thousands of years. These changes in sea level altered Sydney Harbour's coastline and estuary as well as the abundance of marine life available to the Aboriginal people living in the area. During periods of low sea level (when the coastline had long, unbroken sandy beaches with few headlands and no extensive rock platforms and deep estuaries) the abundance of marine life was less than during the period of current high sea level which has existed over the last 6000 years.

Around 18,000 years ago, at the height of the Last Glacial Period, the polar ice caps had enlarged and sea level was at its lowest. At this time, a river ran on the bottom of the Harbour. This river ran out to a sandy coastline which was about 15 kilometres to the east and up to 120 metres below the present sea level.

As the ice-caps melted, the level of the sea rose and spread, flooding the river and turning it into an estuary. However, as the sea level was still rising, this estuary was much smaller than it is today.

The sea reached its present level about 6000 years ago and, apart from minor changes, has remained relatively constant since.

18,000 years ago

11,000 years ago

6,000 years ago