Here’s what you need to know about Sydney’s dam

Thousands of Sydneysiders have been told to be ready to evacuate after Manly Dam threatened to overflow on Tuesday

A red level alert was issued for the dam in Sydney's north, prompting a "get out now" evacuation order for 800 homes in the low-lying areas below

"The rain and thunderstorms have not finished yet," SES Commissioner Carlene York said.

A flood warning for homes and businesses near Narrabeen Lagoon has also been issued, with residents advised to evacuate via Pittwater Road by 7pm.

What happened at Manly?

The dam, located near Manly Vale and Allambie Heights, was built in 1892 to supply water to Manly

and surrounding areas and was repeatedly expanded as demand in the area grew

By 1929, due to rapid population growth, its use as a water source was slowly phased out.

It's now heritage-listed as an example of dam building in the 19th century.