"This rather significant weather system ... we will see it come into the central coast of Sydney and we are already experiencing elements of that right now," Prime Minister Scott Morrison explained during a media briefing.
Sydney is Australia's largest city and home to more than 5 million people. It could receive up to 150 mm (6 inches) of rains within a six-hour period, the Bureau of Meteorology said. (Sydney's mean rainfall for March is 138 mm.)
New South Wales Premier Dominic Perrottet described the extreme weather as a "one-in-a-one thousand-year event" and said emergency crews carried out more than 1,000 rescues in the state after receiving 6,000 calls for help so far.
Hundreds of people are still stuck at their homes in the northern New South Wales city of Lismore, facing its worst floods in history, amid reports of some even spending the night on rooftops.
Mayor Steve Krieg said that nine people were still missing with 400 rescues yet to be carried out.
According to authorities, around 50 people were rescued after they became stuck on a bridge overnight when fast-rising waters submerged both ends.
Australia's east coast summer has been dominated by the La Nina climate pattern (which is typically associated with greater rainfall) for a second straight year.
Brisbane, Australia's third-largest city, received around 80% of its annual rainfall over the last three days alone, Queensland Premier Annastacia Palaszczuk said on Tuesday.
Perrottet said 40,000 people were ordered to evacuate, while 300,000 others were placed under evacuation warnings.
"We'll be doing everything ... we can to get everybody to safety and get these communities right across our state back on their feet as quickly as possible," Perrottet told reporters in Sydney.
Government meteorologist Jonathan Howe described the amount of recent rainfall in northern New South Wales and southern Queensland as "astronomical."
Queensland Police Commissioner Katarina Carroll said emergency services held grave concerns for a man in his 70s who fell from his moored yacht in the state capital Brisbane into a swollen river on Saturday and for a 76-year-old man who disappeared with his vehicle in floodwater northwest of Brisbane on Sunday.
Australia's hottest and driest year on record was 2019 which ended with devastating wildfires across the southeast part of the country. The fires then killed 33 people directly and another 400 people were killed by the smoke.
The fires also destroyed more than 3,000 homes and 19 million hectares (47 million acres ) of farmland and forests.
But two La Nina weather patterns have since brought above-average rainfall to the same regions.
