Dangerous conditions triggered by ex-tropical cyclone Alfred are expected to get worse, with millions of people warned to stay vigilant amid widespread power outages, water rescues and evacuations.

Alfred was downgraded to a tropical low on Saturday morning but severe weather warnings remain for northern NSW and southeast Queensland.

Thousands have been evacuated over flood risks in NSW while all emergency alerts for Queensland were downgraded to Watch and Act early on Saturday afternoon.

The wild weather system is lingering around islands off the coast and is expected to hit the mainland later on Saturday, with the most intense heavy rain and flash flooding still to come.

There have been no reports of deaths or serious injury in Queensland but fears are growing for a man missing after his car was washed away by floodwaters at Megan near Coffs Harbor in NSW.

The 61-year-old local was driving across a flooded bridge when his vehicle was washed off and he had to cling to a tree on Thursday afternoon, NSW detective chief inspector Guy Flaherty told reporters.

He urged people not to drive through floodwaters and said authorities were assessing how to get to the area to locate the man.

“Our concerns for him get worse hour on hour,” he said.

“But obviously we’ve got to look at the safe ability to get in, to access some of these waterways.”

Prime Minister Anthony Albanese said communities were “well prepared” but urged the millions of people under threat to remain alert as conditions intensify.

“It is important that people do not take this downgrading as a reason for complacency,” Mr Albanese told reporters in Canberra.

More than 280,000 homes and businesses lost power in Queensland and NSW, with telecommunications outages in both states.

Six generators from the national stockpile are on their way to Lismore in NSW where a levee is expected to spill later on Saturday.

Queensland Premier David Crisafulli said Gold Coast University Hospital is running on generators after losing power and authorities are racing to get electricity back to two water treatment plants before they run out of water.

An older apartment building on the Gold Coast lost its roof in the wild weather but there is no risk of storm tide inundation along the coast.

Mr Crisafulli told reporters flash flooding and creek rises are the major challenges.

“We have to be ready for that prospect and with already swollen catchments, the risk of flash flooding remains,” he said.

Palm trees in Brisbane were bent sideways in the wind as locals started to venture outside to check the damage on Saturday morning.

Alicia Manning and her husband Jeremie hunkered down at their Wynnum home in the east of Brisbane with friend Otis Matvejs to wait out the cyclone.

They were relieved they wern’t hit with more destructive winds, storm surges or flooding. 

“We thought it was going to be a lot worse,” Ms Manning told AAP.

“It got pretty windy,” Mr Matvejs added.

Gold Coast Council urged residents to stay inside after widespread flooding and significant damage across the city, while public transport in the state’s southeast remains closed.

There were 29 flood rescues in NSW, mainly people driving through flood waters, and the SES said more than 16,000 people were under evacuation orders.

Mr Albanese expected there would be “lasting mental health issues” from the natural disaster and vowed to provide assistance.

Some 180 Australian Defence Force personnel are on the ground in both states with more on standby.

This post was originally published on Michael West.