Australians must not become complacent about the dangers posed by ex-Tropical Cyclone Alfred after it was downgraded to a tropical low weather system on Saturday morning, the prime minister warned.

Tropical Cyclone Alfred was downgraded as it stalled within a few kilometres of the Australian mainland on Saturday morning, but warnings about severe wind and rainfall remain in place across south-east Queensland and northern New South Wales.

A 61-year-old swept off a bridge by fast-moving flood waters near Dorrigo in NSW on Friday remained missing on Saturday. Emergency services were able to briefly talk with the man as he clung to a tree, before he was “unfortunately washed downstream”.

Anthony Albanese has urged people not to drive through flood waters and said it was “important that people do not take this downgrading as a reason for complacency”.

“[This system’s] impact will be serious and will intensify over coming hours and indeed over coming days,” Albanese said. “The impacts are already being felt and there is worse to come in the hours ahead. We must remain vigilant.”

About 250,000 homes in Queensland and 43,000 homes in NSW were without power around noon on Saturday. The Queensland premier, David Crisafulli, said the outages were the “single biggest loss we have seen in over a decade”.

Gold Coast university hospital was without mains power on Saturday morning and was running on generators. Albanese said six generators were being transported to Lismore on Saturday morning.

NSW minister for energy, Penny Sharpe, said it could be “several days” before power was restored in the north of the state.

“You are going to have to be patient,” Sharpe said. “We cannot risk the lives of those workers. But know that we are doing everything we can, as quickly as we can.”

Essential Energy, the state owned electricity infrastructure provider, said debris – including fallen trees and vegetation – would need to be removed before powerlines could be assessed and repaired.

About 740 people in northern NSW had taken refuge across 21 evacuation centres by 10am on Saturday, with 1,110 people registered to use them. About 20,000 people were subject to evacuation warnings in the region.

Emergency services workers in the region conducted 30 flood rescues over 24 hours, with most involving people who had driven through flood waters.

Mick Logan, from the NSW bureau of meteorology, said major flooding was likely in the north of the state from around midday.

Water levels were close to the height of the Lismore CBD levy on Saturday morning, with more than 200mm of rain recorded in the Wilson River catchment area over 24 hours.

River catchments from south-east Queensland to the Nambucca Valley in NSW were already full early on Saturday after days of persistent rain.

The tropical cyclone reached the Moreton Bay islands in the early hours of Saturday, picking up speed but losing some intensity. It was downgraded from a category 2 tropical cyclone to a category 1 at about 1am.

At 6am Alfred was downgraded again by the Bureau of Meteorology, effectively cancelling cyclone warnings – but not other weather warnings – from Noosa to Brisbane. The bureau referred to “ex-Tropical Cyclone Alfred”, which has stalled in Moreton Bay near Bribie Island, about 55km north of Brisbane, and tracked further to the north.

Senior meteorologist at the bureau of meteorology, Miriam Bradbury, said the storm system had not moved position from 6am to shortly before noon.

“We do expect it to make that coastal crossing today,” Bradbury said.

At a press conference on Saturday morning, Crisafulli said wind gusts of more than 100km/h had been recorded on the Gold Coast.

Residents in the region experienced a wild night of strong winds and rain. They have been warned to stay indoors for much of Saturday, and that the “prolonged crossing” could mean that severe wind and rain remain a threat for an extended period.

Matthew Callopy, a senior forecaster at the bureau, said the primary concern was now from heavy rainfall.

“Rainfall totals of over 250mm have already been observed around the Scenic Rim and we’ve seen widespread totals of 100mm to 200mm both on the Gold Coast, but also stretching up into the southern parts of Brisbane,” he said.

“As the remnants of Tropical Cyclone Alfred move inland, we will see more tropical moisture streaming across south-east Queensland and we are expecting widespread totals of 300-500mm, with localised amounts of 800mm-plus possible in some areas of south-east Queensland, particularly again around the southern part of where ex-Tropical Cyclone Alfred tracks.”

Read more of Guardian Australia’s Tropical Cyclone Alfred coverage: