Minor to major flooding occurring from Queensland to Victoria provinces, officials say, warning of tough 24 hours or even 48 hours ahead.
Flood warnings have been sounded across Australia's east coast and tens of thousands of Sydney residents fled their homes overnight as torrential rains again pummelled a large swath of the country.
Bureau of Meteorology forecaster Dean Narramore said on Tuesday minor to major flooding was occurring from the Queensland to Victoria border, a distance of more than 1,555 kilometres.
"A tough 24 hours or even 48 hours ahead," Narramore said during a media briefing on Tuesday as he forecast up to 120 mm (5 inches) of rains across Sydney over the next 24 hours, with the storm clearing by late Wednesday.
Australia's eastern rivers were already near capacity following record downpours in several parts of Queensland and New South Wales states over recent weeks, cutting off towns, and sweeping away farms, livestock and roads.
Nineteen people have been killed, most either in flooded homes or in cars attempting to cross flooded roads, since the deluge began.
"It's very much the watery equivalent of the 'Black Summer' bushfires," emergency services spokesperson Phil Campbell told the AFP news agency.
'No past history similar to this event'
Heavy rains lashed Sydney overnight with some suburbs receiving up to 200 mm since Monday morning, exceeding March's mean rainfall of around 140 mm, triggering flash flooding and snap evacuation orders in the southwest of the city.
Emergency services estimate around 70,000-80,000 people in Greater Sydney face evacuation orders, although they do not expect everyone to follow them.
"People make decisions based on past history and I think this event has shown that there is no past history similar to this event," New South Wales Emergency Service Commissioner Carlene York told reporters.
Frustration was growing among many flood-hit residents in both states over slow relief and recovery efforts, with power and internet still down in several towns as emergency crews tried to clear roads to deliver essential supplies.
Prime Minister Scott Morrison, who is trailing in polls ahead of a federal election due by May, said on Monday more defence force personnel were being sent to flood-affected areas.
Australia has been at the sharp end of climate change, with droughts, deadly bushfires, bleaching events on the Great Barrier Reef and floods becoming more common and intense as global weather patterns change.