Emergency workers were called north of Cessnock in the Hunter region of NSW after reports a Mini car was grounded due to the floods about 8pm on Saturday.
The driver, a 27-year-old woman, managed to get out but the passenger, 26, was dragged away with the strong current.
Police said the women were visiting the Hunter from Sydney, but had decided to return home due to their concern about the area flooding.
"It's a tragic circumstance and a dire situation in terms of a warning for community members," Hunter Valley police superintendent Steve Laksa said.
"We've had a number of areas in the Hunter Valley that have been prone to evacuation orders and warnings in relation to floodwaters. I'd just ask the community to take heed of those warnings and do not enter any waterway."
The missing woman is an engineer and has been in Australia on a working visa for the past three years.
A search for her was suspended overnight and resumed on Sunday morning with multiple emergency services crews taking part.
Meanwhile, State Emergency Service (SES) crews responding to the emergency in the Hunter managed to save a 40-year-old man who was stuck in a nearby tree due to floodwater.
The man was swept out of the tree but rescuers followed him in the water and managed to pull him to shore, and he was taken to hospital.
Both sides of the nation are facing wild weather, with high winds and storms expected in Western Australia and soggy conditions and flooding continuing for northern NSW.
Tens of thousands of households in the eastern state spent a night without power as heavy rain, floods and unprecedented snow wreaked havoc.
The SES has responded to more than 1455 call-outs related to rain, thunderstorms and snow.
Residents in some parts of Taree on the mid north coast have been asked to evacuate due to flooding, while warnings are in place for the Peel and Namoi Rivers at Tamworth, Manilla and Gunnedah.
Armidale and Guyra in New England had unprecedented snowfall and SES crews responded to more than 100 trapped cars.
A new low-pressure system is expected to push up the NSW coast on Sunday, but conditions are likely to ease by afternoon, the Bureau of Meteorology said.
Elsewhere, the bureau has issued a severe weather warning as a cold front moves through WA on Sunday, with damaging winds expected across the state's southwest, affecting Perth, Albany, Bunbury, Margaret River and the goldfields.
"Most of these damaging wind gusts will be driven by showers and thunderstorms," meteorologist Miriam Bradbury said.
Authorities warn surfers, fishers, swimmers and boating enthusiasts to stay clear of the coast amid dangerous swells.
More than 27,500 NSW homes and businesses experienced power disruptions, Essential Energy said on Saturday night.
People have been urged to stay at least eight metres away from fallen power lines, power poles and damaged electricity equipment.
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