Parts of New South Wales have seen almost 1m (3.2ft) of rain, flooding hundreds of homes and severing roads.
Despite the rains subsiding, water levels have not yet peaked in some areas, the Bureau of Meteorology says.
Around 18,000 people in NSW have been displaced.
"It is catastrophic in its dimensions," said NSW Premier Gladys Berejiklian.
Prime Minister Scott Morrison said Western Sydney was now the "greatest concern".
He said Sydney's largest dam would continue to spill over for at least another week.