More than 85,000 people in New South Wales, most in Sydney's western suburbs, have been asked to either evacuate or warned they might receive evacuation orders, up from Tuesday's 50,000, authorities said.
"This still remains a dangerous situation and we need to respond appropriately," Prime Minister Anthony Albanese said during a media briefing, as he declared a one-off emergency cash payment of A$1,000 ($680) for flood-hit residents.
The intense low-pressure system off Australia's east coast moved to the mid-north coast of New South Wales stretching across 300 km (186 miles), with the weather bureau predicting rainfall in excess of 200mm (8 inches) there over six hours.
Torrential rains since Saturday continue to dump waters into river catchments around Sydney, already near full capacity before the latest deluge, as authorities warned the floods crisis could prolong until early next week.
Australia's east coast weather has been dominated by the La Nina phenomenon, typically associated with greater rainfall, two years in a row. The event ended in June, but there is a 50-50 chance it may re-form later this year, according to the Bureau of Meteorology.
Television footage showed cars parked on top of rooftops and residents lining up homes and businesses with sandbags, while emergency crews were seen rescuing stranded farm animals.
Some regions in New South Wales were hit with up to 700 mm (28 inches) of rainfall since Saturday, more than the annual average, but conditions have begun to ease in Sydney.
"We're looking at some dry conditions (in Sydney) tomorrow and then Friday, some slight showers returning on the weekend but nothing quite as heavy as what we have seen," meteorologist Jonathan How told Australian Broadcasting Corp.