Tropical Cyclone Alfred has made a dramatic right turn, and is now “heading towards the Queensland coast”, the state’s premier has warned.

Alfred had been gradually heading in a generally southerly direction for 12 days, but changed course on Tuesday as expected.

“Tropical Cyclone Alfred has just changed direction and is now heading towards the QLD Coast,” the premier, David Crisafulli, said on Tuesday evening on social media.

“We’re prepared for this weather event and we’re asking Queenslanders to do the same.

“Be prepared and stay informed.”

The Brisbane mayor, Adrian Schrinner, warned on Tuesday evening that as many as 20,000 properties could be “impacted” by storm surge or flooding.

“These 20,000 properties could experience anything from mild inundation in their yards to significant flooding in their homes,” Schrinner said.

According to the modelling produced by the Brisbane city council, based on Bureau of Meteorology forecasts, the areas most at risk included Nudgee Beach, Brighton, Windsor, Ashgrove, Morningside and Rocklea.

Schrinner urged residents of those suburbs to consider relocating ahead of the storm.

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Meanwhile the City of Moreton Bay put its community on a watch and act warning on Tuesday evening.

The weather system was now almost certain to be the first cyclone to . The slow-moving storm was expected to hit on either Thursday night or Friday morning.

It was  early on Tuesday.

 on Tuesday evening forecast the storm to hit somewhere between Brisbane and the Sunshine Coast. The most intense rainfall was expected to the south of where Alfred lands.

The bureau warned that more than 4 million people in south-east Queensland and north-eastern  could be potentially affected.

Residents were warned of potential damaging wind gusts of up to 120 kilometres an hour, heavy rainfall causing potential life-threatening flash flooding, and abnormally high tides and damaging surf in coming days.

Crisafulli warned against complacency on Tuesday afternoon.

“One thing I know about Queenslanders is they handle disasters better than anywhere else and we have proven that time and time again,” he said.

“I know the people in the south-east may not have experienced a cyclone before, but they have been tested with floods and they have come through with flying colours every time. And whilst this is a different kind of event, the preparations you put in place make a world of difference and you have proven before that you can do it, and I’m sure that you will again.”

Brisbane has flooded five times, in 1974, 2011, 2022 and twice in 1893; two of them were caused by a cyclone.

Local governments across south-east Queensland had spent the last two days distributing tens of thousands of sandbags.

Brisbane city council, which already resupplied five sandbag depots, opened four more on Tuesday.

Demand still outstripped supply, with some residents reporting waits of up to two hours, or longer.

Josh Humphrey waited for five hours in total, before filling up at the The Whites Hill Reserve in Camp Hill.

“I’ve been sitting in the car waiting for sand since eight this morning,” he said.

“Went to Morningside, gridlock on Wynnum Road for an hour-and-a-half to get to the front gate to say, ‘Oh, we ran out of sand. It’ll be an hour until next truck comes.’ Came here, spent two hours in the queue.”

The acting police commissioner, Shane Chelepy, said there were more than 100,000 sandbags ready to be distributed across the south-east Queensland region on Tuesday, with more coming on Wednesday.

Chelepy said there were an additional 250,000 sandbags ordered to arrive in the next 24 hours.

The prime minister, Anthony Albanese, was expected to make a visit to Brisbane this week, with the treasurer, Jim Chalmers, and emergency management minister, Jenny McAllister, already in the state.

The commonwealth would provide heavy-lift helicopters from the National Aerial Firefighting Centre to assist with recovery.

Guardian Australia understands the defence force had been briefed but not yet called out.

Alfred was 600km east of Brisbane and 570km east of the Gold Coast on Tuesday evening. It was heading south-west at 20km/h.

**Read more of Guardian Australia’s  coverage:**

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