Sydney : More than 20cm (8ins) of rain has fallen over many areas, with some hit by as much as 35cm (14ins) since Saturday.
An intense low-pressure system was forecast to bring more heavy rain to Australia's east coast on Monday with the Bureau of Meteorology predicting another 10cm (4ins).
"The latest information we have is that there's a very good chance that the flooding will be worse than any of the other three floods that those areas had in the last 18 months," Emergency Management Minister Murray Watt told Australian Broadcasting Corp.
Climate change is widely believed to be a contributing factor to the frequent severe weather events, the Climate Council said, adding Australia is "under-prepared".
Dr Ian Wright, from the School of Science at Western Sydney University, said Western Sydney has spread onto flood-prone lands and development has been approved "despite the known risks of flooding".
"Much of the flooding has been impacting vulnerable people. Many cannot afford insurance premiums for flooding. People are exhausted. Many have not recovered from previous flooding," Dr Wright added.