Flood warnings stretched across Australia's east coast on Tuesday and tens of thousands of Sydney residents fled their homes overnight as torrential rains again pummelled the country's largest city, causing flash floods.

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.

A man and a woman were found dead on Tuesday near an abandoned car in a stormwater canal in western Sydney, authorities said, while Queensland police confirmed the death of a man missing in floods since Feb. 27, taking the death tally to 20 since the deluge began. Most people were found dead either in flooded homes or in cars attempting to cross flooded roads.

Bureau of Meteorology forecaster Dean Narramore said minor to major flooding was occurring from the Queensland to Victoria border, a distance of more than 1,555 kilometres (966 miles).

"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 expected to clear by late Wednesday.

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.

Television footage showed flooded roads and homes and stranded cars, as well as the collapse of a city supermarket roof.

Emergency services estimate around 70,000-80,000 people in Greater Sydney face evacuation orders, and urged people 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 as they struggled to clear debris and sludge, with power and internet still down in several towns. Authorities fear even more rain will hamper relief efforts as emergency crews look 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.

New South Wales Premier Dominic Perrottet, on a tour of the flood-hit areas, took responsibility for his government's shortcomings over rescue and relief measures.

"From stories and people I have met, the heartbreaking stories over the course of the week where people felt isolated and abandoned, I don't want anyone in my state to ever feel like that," Perrottet told ABC Radio on Tuesday.